NFS problems with warty and amd64 ?

[lists+ubuntu-users at lists.oddment.net]

So I just got done migrating my fileserver from an old Celeron RH7.3
machine to a new Opteron Warty.  Aside from some hiccups with postfix,
everything seemed fine.  But when I go to mount an NFS volume exported
from the Warty box on either the old RH 7.3 system or a Hoary machine,
things go badly wrong.

After mounting the volume the first time, I did a 'touch foo' in the
mounted directory to make sure things seemed sane, and it worked.  But
trying to do anything other than create files causes problems:

503: ls -l foo
/bin/ls: foo: No such file or directory
504: touch foo
505: ls -l foo
-rw-r--r--    1 user user        0 Feb 28 14:47 foo
506: dd if=/dev/zero of=foo bs=1024 count=1
1+0 records in
1+0 records out
dd: closing output file `foo': Invalid argument
507: ls -l foo
-rw-r--r--    1 user user        0 Feb 28 14:47 foo

I also get error messages on cp and append operations, but nothing in
the log file of either machine.

I'm trying to figure out what's going on.  My mount options on the RH
box are:
fs:/home/user       /home/user          nfs     defaults 0 0

And the export file has:
/home/user          192.168.1.0/24(rw,async)

This is with:
ii  nfs-common     1.0.6-3ubuntu1 NFS support files common to client and serve
ii  nfs-kernel-ser 1.0.6-3ubuntu1 Kernel NFS server support
ii  portmap        5-4ubuntu5     The RPC portmapper

..plus the stock 2.6.8.1-5-amd64-k8-smp kernel + nvnet driver from
nvidia.  The server is an Iwill DK8N + Opteron 242, which I believe is
the nForce 3 chipset.

I've transfered several gig of data over the network link without
any apparent problems, so I don't _think_ it's a network issue.

Any ideas on how I can go about figuring out what's up ?

	Thanks,

	John A.