Trouble booting NFS root

[lukem.freebsd at cse.unsw.edu.au]

I'm trying to set up a small cluster of diskless boxes using FreeBSD 6.1.

So Far, PXE loads pxeboot which loads the kernel.

But the kernel stops in the middle of booting for no apparent reason. The 
last thing on the console is "Timecounters tick every 1.000 msec", which 
usually occurs just before the root partition is mounted.

When I do a traffic dump on the NFS server, I see:

mount /mnt/local/tinny/root
lookup /etc
lookup fstab.split (this fails as there is no /etc/fstab.split)
mount /mnt/local/tinny/root
lookup /etc
lookup /etc/fstab.gz.split (also fails)
mount /mnt/local/tinny/root
lookup /etc
lookup /etc/fstab.gz (also fails)
mount /mnt/local/tinny/root
lookup /etc
lookup /etc/fstab (this time it succeeds)
read from filehandle (contents of /etc/fstab look correct in packet)
second read from the filehandle (returns empty --- I assume this is EOF)

/mnt/local/tinny/fstab contains:
<server>:/mnt/local/tinny/root        /       nfs     ro      0       0
<server>:/mnt/local/tinny/usr         /usr    nfs     ro      0       0

The root partition contains the base and generic kernel from the freebsd 
install cd, and the usr partition is empty for now. (Yes, I'm using the 
GENERIC kernel, though I've also tried it with BOOTP, BOOTP_NFSROOT and 
BOOTP_COMPAT options, which shouldn't, and didn't make any difference).

Questions:
1) Are the reads to fstab from the kernel or the bootloader?
2) Why does the kernel stop at this point, and how can I fix it?

-- 
Luke