Thanks to Manuel for the cpu_reboot incantation, I have managed to get
a core dump from my panic ridden i386 box. Within the wads of backtrace, 
I think the relevant bit is (though could well be wrong!):

#26 0xf01ed290 in trap (frame={tf_es = 16, tf_ds = 16, tf_edi = -263090165, 
      tf_esi = -163295592, tf_ebp = -163173252, tf_ebx = -163172600, 
      tf_edx = 49228, tf_ecx = -163295644, tf_eax = -162541364, tf_trapno = 6, 
      tf_err = 0, tf_eip = -267030924, tf_cs = 8, tf_eflags = 66182, 
      tf_esp = -163210604, tf_ss = 0, tf_vm86_es = -163172600, 
      tf_vm86_ds = 744, tf_vm86_fs = -163295644, tf_vm86_gs = 744})
    at ../../../../arch/i386/i386/trap.c:813
#27 0xf0100c4d in calltrap ()
#28 0xf01d13e1 in ufs_lookup (v=0xf6462d78)
    at ../../../../ufs/ufs/ufs_lookup.c:1151
#29 0xf0159db4 in lookup (ndp=0xf6462ee4) at ../../../../kern/vfs_lookup.c:231
#30 0xf0159ab3 in namei (ndp=0xf6462ee4) at ../../../../kern/vfs_lookup.c:231
#31 0xf0160e4a in vn_open (ndp=0xf6462ee4, fmode=514, cmode=420)
    at ../../../../kern/vfs_vnops.c:570
#32 0xf015d57c in sys_open (p=0xf6368b40, v=0xf6462f88, retval=0xf6462f80)
    at ../../../../kern/vfs_syscalls.c:346
#33 0xf01ed875 in syscall (frame={tf_es = 31, tf_ds = 31, tf_edi = 60, 
      tf_esi = 5, tf_ebp = -272638624, tf_ebx = 1, tf_edx = -272638800, 
      tf_ecx = -272640524, tf_eax = 5, tf_trapno = 3, tf_err = 2, 
      tf_eip = 1208720781, tf_cs = 23, tf_eflags = 582, tf_esp = -272639984, 
      tf_ss = 31, tf_vm86_es = 0, tf_vm86_ds = 0, tf_vm86_fs = 0, 
      tf_vm86_gs = 0}) at ../../../../arch/i386/i386/trap.c:813
#34 0xf0100cf9 in syscall1 ()

Note: this is on elf. I think someone mentioned that => line numbers may
not be 100%  correct..

I'll resup/rebuild now just in case, but the core dump remains, so more
info available if needed!

Cheers,

Patrick