panic running fsck_ufs

Hello, list.

I'm experiencing some weirdness with fsck_ufs after my system became
highly unstable.  The short story is this:

I updated from 7-STABLE to -CURRENT.  After 'shutdown now; make
installworld' I was not able to log into the machine as root at the
console nor through SSH.  As a last resort, I hit the panic button
(power button).

The machine rebooted normally, and I was able to log in again.  After
running mergemaster, I rebooted cleanly and everything seemed okay.
Apparently, fsck_ufs did not like the fact that the previous fsck did
not finish.  To make sure fsck_ufs was the reason my machine bogged
down on me, I was observing the output from 'top' when I was presented
with this:

-- panic start --
orion# dev = ad5s1f, block = 1, fs = /usr
cpuid = 1
KDB: enter: panic
[thread pid 49 tid 100070 ]
Stopped at      kdb_enter+0x3a: movl    $0,kdb_why
db>
-- end panic --

Because this machine is primarily a desktop, have a USB keyboard
attached -- because of that, I cannot access the debugging information
after a panic.  Unfortunately, I do not have access to a PS/2 keyboard
at the moment.

Here's where my concern is though -- this is the output from kgdb:

-- kgdb start --

orion# kgdb kernel.debug /var/crash/vmcore.0
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...
Cannot access memory at address 0x20736968
(kgdb) backtrace
#0  0x00000000 in ?? ()
(kgdb)

 -- kgdb end --

If my understanding of this is correct, there is nothing in the stack?
 Is this because fsck_ufs is running live?

The machine was updated to the latest HEAD from SVN approximately an
hour and a half before the timestamp in the 'uname -a' output:

FreeBSD orion 8.0-CURRENT FreeBSD 8.0-CURRENT #4 r192066M: Wed May 13
19:48:34 EDT 2009 root_at_orion:/usr/obj/usr/src/sys/GENERIC i386

If this is user error, I'll admit it -- especially since a forceful
power-off is why fsck_ufs is running in the first place.  My concern
is no memory location in the dump.

Any insight would be helpful in figuring out this panic.  (FWIW, the
machine panicked twice while I was writing the email, so it is
completely reproducible.)

-- 
Glen Barber