FS hang when creating snapshots on a UFS SU+J setup

I have a RELENG_9 machine that hangs when a snapshot is created on the
root fs (UFS, with SU+J).  More accurately, all the processes show a
state of "suspfs" (with ^T) and no fs activity is completed from then
on.  A hard reboot (power cycle) was the only way to proceed.

Here's some reference info - let me know what else I should provide.

$uname -a
FreeBSD xxx.xxx.net 9.0-PRERELEASE FreeBSD 9.0-PRERELEASE #0: Sun Dec
25 05:04:37 UTC 2011     root_at_xxx.xxx.net:/usr/obj/usr/src/sys/GENERIC
amd64

csup was run just before build[world|kernel] so you have reference on
the version information.

$mount
/dev/gpt/root on / (ufs, local, journaled soft-updates)
devfs on /dev (devfs, local, multilabel)
linprocfs on /compat/linux/proc (linprocfs, local)
{ zfs info removed }

$df -h
Filesystem                  Size    Used   Avail Capacity  Mounted on
/dev/gpt/root               454G    9.1G    409G     2%    /
devfs                       1.0k    1.0k      0B   100%    /dev
linprocfs                   4.0k    4.0k      0B   100%    /compat/linux/proc
{ zfs info removed }

After the hard reset, there was a snapshot file listed in /.snap and
it was ~465 GB, iirc.  Unfortunately, I needed to get things going
again so I was not able to debug or diagnose further.  I may be able
to schedule a time that I could recreate the issue and diagnose
better, but I wanted to get your input on what data points and/or
command you would be interested in.

Thanks in advance,

Bryce