More precisely: We use a small thin client as a firewall, which boots off the network. Originally, I planned a three-legged firewall with a DMZ, but since the system has only one PCI connector, I was forced to try a dual-port network card.
Now, OpenBSD recognizes both network interfaces fine - fxp0 and fxp1. But as soon as I try to bring one interface up (no matter if link is present or not), the system completely freezes after about one second. No panic, no debugger prompt. It turns into a brick. I have no idea what the cause might be, even tried to fire up systat/vmstat directly after the initialization to see if some kind of IRQ flood might be the cause.
Of course, the NIC runs fine in other systems (separate hardware) - tested with Windows, Linux and Solaris.
Currently the system runs with a single-port xl(4) card (3Com 3c905C 100Base-TX) without any problems and has been stable for days now.
Well, my last hope lies here in the list. I'll try to provide as much information as possible:
1. The NIC
Intel EtherExpress PRO/100 Dual-Port PCI (as mentioned in the fxp(4) man page)

The card has three chips: two Intel SB82558B (NIC), one Intel 21152-AB (PCI bridge) - if that may help in further identification.
2. dmesg
OpenBSD 4.3 (GENERIC) #698: Wed Mar 12 11:07:05 MDT 2008

    deraadt@i386.openbsd.org:/usr/src/sys/arch/i386/compile/GENERIC

cpu0: Geode(TM) Integrated Processor by National Semi ("Geode by NSC" 586-class) 301 MHz

cpu0: FPU,TSC,MSR,CX8,CMOV,MMX

cpu0: TSC disabled

real mem  = 31551488 (30MB)

avail mem = 20885504 (19MB)

mainbus0 at root

bios0 at mainbus0: AT/286+ BIOS, date 05/11/01, BIOS32 rev. 0 @ 0xfb1b0, SMBIOS rev. 2.2 @ 0xf0800 (42 entries)

bios0: vendor Award Software International, Inc. version "4.51 PG" date 05/11/01

apm0 at bios0: Power Management spec V1.2

apm0: AC on, battery charge unknown

acpi at bios0 function 0x0 not configured

pcibios0 at bios0: rev 2.1 @ 0xf0000/0xb638

pcibios0: PCI IRQ Routing Table rev 1.0 @ 0xfdf60/64 (2 entries)

pcibios0: PCI Exclusive IRQs: 11 15

pcibios0: no compatible PCI ICU found: ICU vendor 0x1078 product 0x0100

pcibios0: Warning, unable to fix up PCI interrupt routing

pcibios0: PCI bus #1 is the last bus

bios0: ROM list: 0xc0000/0x8000 0xc8000/0xa000

cpu0 at mainbus0

pci0 at mainbus0 bus 0: configuration mode 1 (no bios)

pchb0 at pci0 dev 0 function 0 "Cyrix GXm PCI" rev 0x00

ppb0 at pci0 dev 7 function 0 "DEC 21152 PCI-PCI" rev 0x03

pci1 at ppb0 bus 1

fxp0 at pci1 dev 4 function 0 "Intel 8255x" rev 0x05, i82558: irq 15, address 00:90:27:e8:bb:65

inphy0 at fxp0 phy 1: i82555 10/100 PHY, rev. 0

fxp1 at pci1 dev 5 function 0 "Intel 8255x" rev 0x05, i82558: irq 11, address 00:90:27:e8:bb:66

inphy1 at fxp1 phy 1: i82555 10/100 PHY, rev. 0rl0 at pci0 dev 15 function 0 "Realtek 8139" rev 0x10: irq 11, address 00:e0:c5:bc:24:a1

rlphy0 at rl0 phy 0: RTL internal PHY

pcib0 at pci0 dev 18 function 0 "Cyrix Cx5530 South" rev 0x30

"Cyrix Cx5530 SMI" rev 0x00 at pci0 dev 18 function 1 not configured

pciide0 at pci0 dev 18 function 2 "Cyrix Cx5530 IDE" rev 0x00: no DMA, channel 0 wired to compatibility, channel 1 wired to compatibility

pciide0: channel 0 ignored (not responding; disabled or no drives?)

pciide0: no compatibility interrupt for use by channel 1

"Cyrix Cx5530 XpressAUDIO" rev 0x00 at pci0 dev 18 function 3 not configured

vga1 at pci0 dev 18 function 4 "Cyrix Cx5530 Video" rev 0x00

wsdisplay0 at vga1 mux 1: console (80x25, vt100 emulation)

wsdisplay0: screen 1-5 added (80x25, vt100 emulation)

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

pcppi0 at isa0 port 0x61

midi0 at pcppi0:

spkr0 at pcppi0

npx0 at isa0 port 0xf0/16: reported by CPUID; using exception 16

biomask 77fd netmask fffd ttymask ffff

softraid0 at root

PXE boot MAC address 00:e0:c5:bc:24:a1, interface rl0

nfs_boot: using interface rl0, with revarp & bootparams

nfs_boot: client_addr=172.16.0.2

nfs_boot: server_addr=172.16.0.1 hostname=igel

root on 172.16.0.1:/srv/igel/root

swap on 172.16.0.1:/srv/igel/swap
PS: pcibios0: no compatible PCI ICU found: ICU vendor 0x1078 product 0x0100 - if this also caught your attention and you need further information from me regarding this message, I am more than willing to help and cooperate (apologies... no sufficient coding skills).