There were reports about panics when a diskless
system is shut down after "serious activity".
I just got such a panic, and since I don't remember
to have seen a traceback, I'll post it here.
(I HAVE to start weekend now, otherwise I would
try to track down further...)

cpu_reboot()->vfs_shutdown()->vfs_unmountall
->dounmount()->nfs_unmount()->vflush()
Here vgone() is called. The compiler does some
common code elimination here - there is only 1
call in the binary. vp->v_usecount is 3 in my case,
so I assume it is the second call in the source.
->vclean()->nfs_inactive()->nfs_removeit()
->nfs_removerpc()
The "v3 = NFS_ISV3(dvp);" at the beginning
causes the panic because the v_mount member
of "dvp" is 0.

Hope this helps.

best regards
Matthias Drochner