I've seen this problem myself - perhaps I could add more info that
James could confirm/deny..  I've seen this in multi-user mode &
noticed that when this happens, my shell (tcsh) & its parents
(rlogind, inetd) get stuck in D-waits.  I was able to figure this out
because I was coming in over the network.  I was able to log in via a
serial console & get this information.

It does seem related to the amount of memory in a machine.  I'm
running with 16M & this doesn't happen to me every time I rebuild a
kernel.. 

I haven't had a chance to sup since mid-march, so I can't say if this
is still happening in the most recent kernels..

Drew