Good time of day,

-------------------------
orphan# uname -a
OpenBSD orphan.oganer.net 4.2 GENERIC#0 i386
orphan# reboot
/etc/rc.shutdown in progress...
/etc/rc.shutdown complete.
syncing disks... done
rebooting...
*HANG*
-------------------------

same on latest snapshot kernel (w/ -stable userland)
halt(8) works fine,
strl-Alt-Del (before kernel boot) works fine
BIOS Setup defaults reset does not help

some kernel debugging:
printf() control points on cpu_reset():
-------------------------
syncing disks... done
rebooting...
cpu_reset(): disabling IRQs ...
cpu_reset(): calling (*cpuresetfn)() ...
cpu_reset(): hacking keyboard ...
*HANG*
-------------------------

comment out keyboard controller code:
-------------------------
syncing disks... done
rebooting...
cpu_reset(): disabling IRQs ...
cpu_reset(): calling (*cpuresetfn)() ...
cpu_reset(): hacking keyboard (disabled) ...
cpu_reset(): causing triple fault by IDT ...
*HANG*
-------------------------

comment out IDT invalidation code:
-------------------------
syncing disks... done
rebooting...
cpu_reset(): disabling IRQs ...
cpu_reset(): calling (*cpuresetfn)() ...
cpu_reset(): hacking keyboard (disabled) ...
cpu_reset(): causing triple fault by IDT (disabled) ...
cpu_reset(): causing triple fault by address space unmap ...
cpu_reset(): entering infinite loop ...
*HANG*
-------------------------

Any clue ?
Faulty hardware ? (looking for identical hardware to test)

Kind regards,

Ilya A. Kovalenko                  (mailto:shadow@oganer.net)
SpecialEQ SW section
JSC Oganer-Service

P.S. dmesg (latest snapshot):

OpenBSD 4.2-current (GENERIC) #607: Tue Dec 18 18:36:52 MST 2007
    deraadt@i386.openbsd.org:/usr/src/sys/arch/i386/compile/GENERIC
cpu0: Intel(R) Celeron(R) CPU 2.53GHz ("GenuineIntel" 686-class) 2.54 GHz
cpu0: FPU,V86,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,TM2,CNXT-ID,CX16,xTPR
real mem  ...
[ message continues ]

If you use acpi with -current it should reboot via acpi.

Try disable apm via doing the following at the boot prompt.
boot -c
disable apm
quit

This can be made more permanent via config(8).