I ran across this last night, and noticed to my dismay that I had /usr
mounted _twice_.  As soon as I commented out the mounts of specific
file systems other than root, I was able to reboot successfully.  Note
that in my case, I couldn't get a clean reboot at any point while
/usr was mounted twice.

I'm not sure what the solution is, but one thing that particularly
troubles me is the potential for `mount /usr' to be performed twice in
/etc/rc, besides the `mount -a' that is invoked later on.

--scott