Quoting Michael L. VanLoon -- HeadCandy.com,

> Running current supped Sept. 5th.  Machine was running fine.  I just
> rebuilt the entire system and installed, so was going to reboot.
> Typed "shutdown -r now".  Waited...  Came back a little later and
> looked on the console and saw:

> 	syncing disks... panic: vinvalbuf: dirty bufs

This happens when you install a new /sbin/init (while the old one is 
still running -- this is kind of hard to avoid ;-)).

It is a known problem, there is a send-pr of it by cgd somewhere I think.

- Frank