When trying to reboot, I got:

syncing disks... 12 12 5 done
uvm_vnp_terminate(0xf0539800): warning: object still active with 2 refs
uvm_vnp_terminate: flags = 0x1, refs=2
panic: uvm_vnp_terminate: uvn in unexpected state

Pentium, 64MB RAM, UVM kernel from Feb 17 sources. I don't always get a
panic though; most of the time when I reboot, I get something like:

uvm_vnp_terminate(0xf0593300): warning: object still active with 2 refs
uvm_vnp_terminate(0xf04ace00): warning: object still active with 2 refs

and then it reboots.

core dump and kernel with debug symbols available...
-- 
Name: Dave Huang     |   Mammal, mammal / their names are called /
INet: khym@bga.com   |   they raise a paw / the bat, the cat /
FurryMUCK: Dahan     |   dolphin and dog / koala bear and hog -- TMBG
Dahan: Hani G Y+C 22 Y++ L+++ W- C++ T++ A+ E+ S++ V++ F- Q+++ P+ B+ PA+ PL++