As reported by Dennis Nedry (http://www.nabble.com/OpenBSD-4.4-(Generic)-AMD64--
-Missing-ram-above-3GB-td20276366.html) I also face the problem that
OpenBSD AMD64 (and i386) do miss memory on a Thinkpad X61s with 4GB of RAM
and the latest BIOS installed.

If there are any patches avaiable I could test please do let me know.
I did not saw a BIOS option so far which could be responseable for the
missing 1GB of RAM. Furthermore the BIOS shows all 4GB of memory.

Kind regards,
Sebastian

1. dmesg of the thinkpad x61s
2. dmesg of another openbsd-amd64 box which does see more then 3GB of ram

OpenBSD 4.5 (GENERIC.MP) #2133: Sat Feb 28 15:02:16 MST 2009
    deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 3210412032 (3061MB)
avail mem = 3101503488 (2957MB)
mainbus0 at root
bios0 at mainbus0: SMBIOS rev. 2.4 @ 0xe0010 (63 entries)
bios0: vendor LENOVO version "7NETB9WW (2.19 )" date 11/27/2008
bios0: LENOVO 766655G
acpi0 at bios0: rev 2
acpi0: tables DSDT FACP SSDT ECDT TCPA APIC MCFG HPET SLIC BOOT ASF! SSDT SSDT SSDT SSDT
acpi0: wakeup devices LID_(S3) SLPB(S3) DURT(S3) IGBE(S4) EXP0(S4) EXP1(S4) EXP2(S4) EXP3(S4) EXP4(S4) PCI1(S4) USB0(S3) USB1(S3) USB2(S3) USB3(S3) USB4(S3) EHC0(S3) EHC1(S3) HDEF(S4)
acpitimer0 at acpi0: 3579545 Hz, 24 bits
acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat
cpu0 at mainbus0: apid 0 (boot processor)
cpu0: Intel(R) Core(TM)2 Duo CPU L7500 @ 1.60GHz, 1596.27 MHz
cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,VMX,EST,TM2,CX16,xTPR,NXE,LONG
cpu0: 4MB 64b/line 16-way L2 cache
cpu0: apic clock running at 199MHz
cpu1 at mainbus0: apid 1 (application processor)
cpu1: Intel(R) Core(TM)2 Duo CPU L7500 @ 1.60GHz, 1596.00 MHz
cpu1: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,VMX,EST,TM2,CX16,xTPR,NXE,LONG
cpu1: 4MB 64b/line 16-way L2 cache
ioapic0 at mainbus0 apid 1 pa 0xfec00000, version 20, 24 pins
ioapic0: misconfigured as apic 2, remapped to apid 1
acpihpet0 at acpi0: 14318179 Hz
acpiprt0 at acpi0: bus 0 (PCI0)
acpiprt1 at acpi0: bus -1 (AGP_)
acpiprt2 at acpi0: bus 2 (EXP0)
acpiprt3 at acpi0: bus 3 (EXP1)
acpiprt4 at acpi0: bus -1 (EXP2)
acpiprt5 at acpi0: bus -1 (EXP3)
acpiprt6 at acpi0: bus -1 (EXP4)
acpiprt7 at acpi0: bus 5 (PCI1)
acpiec0 at acpi0
acpicpu0 at acpi0: C3, C2
acpicpu1 at acpi0: C3, C2
acpitz0 at acpi0: critical temperature 127 degC
acpitz1 at acpi0: critical temperature 100 degC
acpibtn0 at acpi0: LID_
acpibtn1 at acpi0: SLPB
acpibat0 at acpi0: BAT0 model "42T4568" serial  4165 type LION oem "SONY"
acpibat1 at acpi0: BAT1 not present
acpibat2 at acpi0: BAT2 not present
acpiac0 at acpi0: AC unit online
acpithinkpad0 at acpi0
acpidock at acpi0 not configured
acpivideo at acpi0 not configured
acpivideo at acpi0 not configured
cpu0: unknown Enhanced SpeedStep CPU, msr 0x0615081906000819
cpu0: using only highest and lowest power states
cpu0: Enhanced SpeedStep 1600 MHz (1100 mV): speeds: 1600, 1200 MHz
pci0 at mainbus0 bus 0: configuration mode 1
pchb0 at pci0 dev 0 function 0 "Intel GM965 Host" rev 0x0c
vga1 at pci0 dev 2 function 0 "Intel GM965 Video" rev 0x0c
wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)
wsdisplay0: screen 1-5 added (80x25, vt100 emulation)
intagp0 at vga1
agp0 at intagp0: aperture at 0xe0000000, size 0x10000000
inteldrm0 at vga1: apic 1 int 16 (irq 10)
drm0 at inteldrm0
"Intel GM965 Video" rev 0x0c at pci0 dev 2 function 1 not configured
em0 at pci0 dev 25 function 0 "Intel ICH8 IGP M AMT" rev 0x03: apic 1 int 20 (irq 11), address 
uhci0 at pci0 dev 26 function 0 "Intel 82801H USB" rev 0x03: apic 1 int 20 (irq 11)
uhci1 at pci0 dev 26 function 1 "Intel 82801H USB" rev 0x03: apic 1 int 21 (irq 11)
ehci0 at pci0 dev 26 function 7 "Intel 82801H USB" rev 0x03: apic 1 int 22 (irq 11)
usb0 at ehci0: USB revision 2.0
uhub0 at usb0 "Intel EHCI root hub" rev 2.00/1.00 addr 1
azalia0 at pci0 dev 27 function 0 "Intel 82801H HD Audio" rev 0x03: apic 1 int 17 (irq 11)
azalia0: RIRB time out
azalia0: codecs: Analog Devices AD1984, Conexant/0x2bfa, using Analog Devices AD1984
audio0 at azalia0
ppb0 at pci0 dev 28 function 0 "Intel 82801H PCIE" rev 0x03: apic 1 int 20 (irq 11)
pci1 at ppb0 bus 2
ppb1 at pci0 dev 28 function 1 "Intel 82801H PCIE" rev 0x03: apic 1 int 21 (irq 11)
pci2 at ppb1 bus 3
iwn0 at pci2 dev 0 function 0 "Intel Wireless WiFi Link 4965AGN" rev 0x61: apic 1 int 17 (irq 11), MIMO 2T3R, MoW2, address 
uhci2 at pci0 dev 29 function 0 "Intel 82801H USB" rev 0x03: apic 1 int 16 (irq 10)
uhci3 at pci0 dev 29 function 1 "Intel 82801H USB" rev 0x03: apic 1 int 17 (irq 11)
ehci1 at pci0 dev 29 function 7 "Intel 82801H USB" rev 0x03: apic 1 int 19 (irq 11)
usb1 at ehci1: USB revision 2.0
uhub1 at usb1 "Intel EHCI root hub" rev 2.00/1.00 addr 1
ppb2 at pci0 dev 30 function 0 "Intel 82801BAM Hub-to-PCI" rev 0xf3
pci3 at ppb2 bus 5
cbb0 at pci3 dev 0 function 0 "Ricoh 5C476 CardBus" rev 0xba: apic 1 int 16 (irq 10)
"Ricoh 5C832 Firewire" rev 0x04 at pci3 dev 0 function 1 not configured
sdhc0 at pci3 dev 0 function 2 "Ricoh 5C822 SD/MMC" rev 0x21: apic 1 int 18 (irq 11)
sdmmc0 at sdhc0
cardslot0 at cbb0 slot 0 flags 0
cardbus0 at cardslot0: bus 6 device 0 cacheline 0x8, lattimer 0xb0
pcmcia0 at cardslot0
pcib0 at pci0 dev 31 function 0 "Intel 82801HEM LPC" rev 0x03
pciide0 at pci0 dev 31 function 1 "Intel 82801HBM IDE" rev 0x03: DMA, channel 0 configured to compatibility, channel 1 configured to compatibility
pciide0: channel 0 disabled (no drives)
pciide0: channel 1 ignored (disabled)
ahci0 at pci0 dev 31 function 2 "Intel 82801HBM AHCI" rev 0x03: apic 1 int 16 (irq 10), AHCI 1.1
scsibus0 at ahci0: 32 targets
sd0 at scsibus0 targ 0 lun 0: <ATA, WDC WD3200BEVT-2, 11.0> SCSI3 0/direct fixed
sd0: 305245MB, 512 bytes/sec, 625142448 sec total
ichiic0 at pci0 dev 31 function 3 "Intel 82801H SMBus" rev 0x03: apic 1 int 23 (irq 11)
iic0 at ichiic0
usb2 at uhci0: USB revision 1.0
uhub2 at usb2 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb3 at uhci1: USB revision 1.0
uhub3 at usb3 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb4 at uhci2: USB revision 1.0
uhub4 at usb4 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb5 at uhci3: USB revision 1.0
uhub5 at usb5 "Intel UHCI root hub" rev 1.00/1.00 addr 1
isa0 at pcib0
isadma0 at isa0
pckbc0 at isa0 port 0x60/5
pckbd0 at pckbc0 (kbd slot)
pckbc0: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard, using wsdisplay0
pms0 at pckbc0 (aux slot)
pckbc0: using irq 12 for aux slot
wsmouse0 at pms0 mux 0
pcppi0 at isa0 port 0x61
midi0 at pcppi0: <PC speaker>
spkr0 at pcppi0
aps0 at isa0 port 0x1600/31
mtrr: Pentium Pro MTRR support
ugen0 at uhub2 port 2 "STMicroelectronics Biometric Coprocessor" rev 1.00/0.01 addr 2
softraid0 at root
root on sd0a swap on sd0b dump on sd0b

-- BOX 2 --

OpenBSD 4.5 (GENERIC.MP) #2129: Thu Feb 26 13:01:54 MST 2009
    deraadt@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP
real mem = 3747151872 (3573MB)
avail mem = 3624951808 (3457MB)
mainbus0 at root
bios0 at mainbus0: SMBIOS rev. 2.5 @ 0xf06b0 (57 entries)
bios0: vendor American Megatrends Inc. version "0409" date 09/11/2008
bios0: ASUSTeK Computer INC. P5KPL-AM
acpi0 at bios0: rev 2
acpi0: tables DSDT FACP APIC MCFG OEMB HPET GSCI
acpi0: wakeup devices P0P2(S4) P0P1(S4) PS2K(S4) PS2M(S4) UAR1(S4) MC97(S4) P0P4(S4) P0P5(S4) P0P6(S4) P0P7(S4) P0P8(S4) P0P9(S4) USB0(S4) USB1(S4) USB2(S4) USB3(S4) EUSB(S4) SLPB(S4)
acpitimer0 at acpi0: 3579545 Hz, 24 bits
acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat
cpu0 at mainbus0: apid 0 (boot processor)
cpu0: Pentium(R) Dual-Core CPU E5200 @ 2.50GHz, 2915.11 MHz
cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,EST,TM2,CX16,xTPR,NXE,LONG
cpu0: 2MB 64b/line 8-way L2 cache
cpu0: apic clock running at 233MHz
cpu1 at mainbus0: apid 1 (application processor)
cpu1: Pentium(R) Dual-Core CPU E5200 @ 2.50GHz, 2914.73 MHz
cpu1: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,SBF,SSE3,MWAIT,DS-CPL,EST,TM2,CX16,xTPR,NXE,LONG
cpu1: 2MB 64b/line 8-way L2 cache
ioapic0 at mainbus0 apid 2 pa 0xfec00000, version 20, 24 pins
acpihpet0 at acpi0: 14318179 Hz
acpiprt0 at acpi0: bus 0 (PCI0)
acpiprt1 at acpi0: bus 3 (P0P1)
acpiprt2 at acpi0: bus 2 (P0P4)
acpiprt3 at acpi0: bus 1 (P0P5)
acpiprt4 at acpi0: bus -1 (P0P6)
acpicpu0 at acpi0
acpicpu1 at acpi0
acpibtn0 at acpi0: SLPB
acpibtn1 at acpi0: PWRB
pci0 at mainbus0 bus 0: configuration mode 1
pchb0 at pci0 dev 0 function 0 "Intel 82G33 Host" rev 0x10
vga1 at pci0 dev 2 function 0 "Intel 82G33 Video" rev 0x10
wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)
wsdisplay0: screen 1-5 added (80x25, vt100 emulation)
intagp0 at vga1
agp0 at intagp0: aperture at 0xe0000000, size 0x10000000
inteldrm0 at vga1: apic 2 int 16 (irq 10)
drm0 at inteldrm0
ppb0 at pci0 dev 28 function 0 "Intel 82801GB PCIE" rev 0x01: apic 2 int 16 (irq 10)
pci1 at ppb0 bus 2
ppb1 at pci0 dev 28 function 1 "Intel 82801GB PCIE" rev 0x01: apic 2 int 17 (irq 11)
pci2 at ppb1 bus 1
re0 at pci2 dev 0 function 0 "Realtek 8101E" rev 0x02: RTL8102EL (0x2480), apic 2 int 17 (irq 11), address 
rlphy0 at re0 phy 7: RTL8201L 10/100 PHY, rev. 1
uhci0 at pci0 dev 29 function 0 "Intel 82801GB USB" rev 0x01: apic 2 int 23 (irq 5)
uhci1 at pci0 dev 29 function 1 "Intel 82801GB USB" rev 0x01: apic 2 int 19 (irq 11)
uhci2 at pci0 dev 29 function 2 "Intel 82801GB USB" rev 0x01: apic 2 int 18 (irq 3)
uhci3 at pci0 dev 29 function 3 "Intel 82801GB USB" rev 0x01: apic 2 int 16 (irq 10)
ehci0 at pci0 dev 29 function 7 "Intel 82801GB USB" rev 0x01: apic 2 int 23 (irq 5)
usb0 at ehci0: USB revision 2.0
uhub0 at usb0 "Intel EHCI root hub" rev 2.00/1.00 addr 1
ppb2 at pci0 dev 30 function 0 "Intel 82801BA Hub-to-PCI" rev 0xe1
pci3 at ppb2 bus 3
xl0 at pci3 dev 0 function 0 "3Com 3c905C 100Base-TX" rev 0x74: apic 2 int 19 (irq 11), address 
bmtphy0 at xl0 phy 24: 3C905C internal PHY, rev. 6
xl1 at pci3 dev 1 function 0 "3Com 3c905C 100Base-TX" rev 0x74: apic 2 int 16 (irq 10), address 
bmtphy1 at xl1 phy 24: 3C905C internal PHY, rev. 6
pcib0 at pci0 dev 31 function 0 "Intel 82801GB LPC" rev 0x01
pciide0 at pci0 dev 31 function 1 "Intel 82801GB IDE" rev 0x01: DMA, channel 0 configured to compatibility, channel 1 configured to compatibility
pciide0: channel 0 disabled (no drives)
pciide0: channel 1 disabled (no drives)
pciide1 at pci0 dev 31 function 2 "Intel 82801GB SATA" rev 0x01: DMA, channel 0 configured to native-PCI, channel 1 configured to native-PCI
pciide1: using apic 2 int 19 (irq 11) for native-PCI interrupt
wd0 at pciide1 channel 0 drive 0: <SAMSUNG HD103UJ>
wd0: 16-sector PIO, LBA48, 953869MB, 1953525168 sectors
wd1 at pciide1 channel 0 drive 1: <SAMSUNG HD103UJ>
wd1: 16-sector PIO, LBA48, 953869MB, 1953525168 sectors
wd0(pciide1:0:0): using PIO mode 4, Ultra-DMA mode 5
wd1(pciide1:0:1): using PIO mode 4, Ultra-DMA mode 5
ichiic0 at pci0 dev 31 function 3 "Intel 82801GB SMBus" rev 0x01: apic 2 int 19 (irq 11)
iic0 at ichiic0
spdmem0 at iic0 addr 0x50: 2GB DDR2 SDRAM non-parity PC2-6400CL5
spdmem1 at iic0 addr 0x52: 2GB DDR2 SDRAM non-parity PC2-6400CL5
usb1 at uhci0: USB revision 1.0
uhub1 at usb1 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb2 at uhci1: USB revision 1.0
uhub2 at usb2 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb3 at uhci2: USB revision 1.0
uhub3 at usb3 "Intel UHCI root hub" rev 1.00/1.00 addr 1
usb4 at uhci3: USB revision 1.0
uhub4 at usb4 "Intel UHCI root hub" rev 1.00/1.00 addr 1
isa0 at pcib0
isadma0 at isa0
com0 at isa0 port 0x3f8/8 irq 4: ns16550a, 16 byte fifo
pckbc0 at isa0 port 0x60/5
pckbd0 at pckbc0 (kbd slot)
pckbc0: using irq 1 for kbd slot
wskbd0 at pckbd0: console keyboard, using wsdisplay0
pcppi0 at isa0 port 0x61
midi0 at pcppi0: <PC speaker>
spkr0 at pcppi0
lpt0 at isa0 port 0x378/4 irq 7
wbsio0 at isa0 port 0x2e/2: W83627DHG rev 0x23
lm1 at wbsio0 port 0x290/8: W83627DHG
mtrr: Pentium Pro MTRR support
uhub5 at uhub2 port 2 "Cherry USB-Hub" rev 1.10/2.13 addr 2
ugen0 at uhub5 port 3 "OMNIKEY AG Smart Card Reader USB" rev 1.10/2.00 addr 3
uhidev0 at uhub5 port 4 configuration 1 interface 0 "Cherry USB-Keyboard" rev 1.10/2.13 addr 4
uhidev0: iclass 3/1
ukbd0 at uhidev0: 8 modifier keys, 6 key codes
wskbd1 at ukbd0 mux 1
wskbd1: connecting to wsdisplay0
softraid0 at root
root on wd0a swap on wd0b dump on wd0b