5.1-R-p11 unable to "sync disks" when shutting down

For some reason my home PC is occasionally unable to sync to disk when I
run "shutdown -p now". The HW in question is a Shuttle SN45G which has
NVIDIA nForce2 Ultra 400 North bridge and MCP-T South bridge. I've
attached dmesg to this e-mail.

I managed to activate kernel debugger with ctrl-alt-esc, but I was
little bit late so "sync disks" had already given up. Then I ran "panic"
command from debugger so I would have a dump which might help to find
out why this happened.

The only hard disk has four slices. ad0s1 has w2k, ad0s2 has 5.2-BETA,
ad0s3 has 5.1-R-p11 and ad0s4 has /home, /usr/ports and /scratch
filesystems. When installing 5.2-BETA I managed to lose (due to my
error) ad0s3 and ad0s4 disklabels. The ad0s4 (which has only UFS1
filesystems) disklabel was reconstructed by using Niall Smart's "findsb"
and I had a copy of the ad0s3 disklabel at my $HOME dir.