With a kernel supped on Tuesday 17th, i386:

panic: pfil_list_remove
#0  0xf021d928 in devcls ()
(gdb) bt
#0  0xf021d928 in devcls ()
#1  0x4fcd000 in ?? ()
#2  0xf01e8783 in cpu_reboot (howto=256, bootstr=0x0)
    at ../../../../arch/i386/i386/machdep.c:1212
#3  0xf0143b19 in panic () at ../../../../kern/subr_prf.c:217
#4  0xf0170322 in pfil_list_remove (list={tqh_first = 0x0, 
      tqh_last = 0xf0533e40}, func=0xf018dac0 <fr_check>)
    at ../../../../net/pfil.c:152
#5  0xf01702c0 in pfil_remove_hook (func=0xf018dac0 <fr_check>, flags=3)
    at ../../../../net/pfil.c:129
#6  0xf018c185 in ipl_disable () at ../../../../netinet/ip_fil.c:303
#7  0xf018c459 in iplioctl (dev=11264, cmd=2147775048, data=0xf64a8ec4 "", 
    mode=3, p=0xf6470644) at ../../../../netinet/ip_fil.c:419
#8  0xf01644ae in spec_ioctl (v=0xf64a8df4)
    at ../../../../miscfs/specfs/spec_vnops.c:423
#9  0xf0161d3e in vn_ioctl (fp=0xf63798d0, com=2147775048, data=0xf64a8ec4 "", 
    p=0xf6470644) at ../../../../sys/vnode_if.h:327
#10 0xf0146231 in sys_ioctl (p=0xf6470644, v=0xf64a8f88, retval=0xf64a8f80)
    at ../../../../kern/sys_generic.c:637
#11 0xf01ee5e1 in syscall (frame={tf_es = 31, tf_ds = 31, tf_edi = -272637968, 
      tf_esi = 2, tf_ebp = -272638840, tf_ebx = -272638748, 
      tf_edx = -272638832, tf_ecx = -272638652, tf_eax = 54, tf_trapno = 3, 
      tf_err = 2, tf_eip = 134629969, tf_cs = 23, tf_eflags = 535, 

The computer was running with no ipf rules, and 2 simple ipnat ones. I think
the sequence before the panic was
ipf -D
ipnat -f /etc/ipnat.conf
but I'm not sure. (Don't think it got as far as ipf -E)

Any thoughts on extracting more useful information from the coredump?

Cheers,

Patrick