I just bought a DNS-321 and set it up on my LAN. I have, at this point, an iMac and a MacMini, both running OSX 10.6.4 (the MacMini is running the server version) and two Linux boxen, both running Debian. The other systems are virtual (through VirtualBox, Xen, or Parallels), but they're not part of the issue and none have been running for a while.
I've included a screenshot of my NFS settings:

In short:
Host: 172.16.7.0/255.255.255.0
Path: Volume_1/threshNet
Real Path: /mnt/HD_a2/threshNet
Root Squash: Yes
(I've also tried 172.16.7.0/24. The DNS-321 is at 172.16.7.99 and this system is 172.16.7.4.)
Every time I try something with /Volume_1/threshNet I also try it with /mnt/HD_a1/threshNet as well.
And then I've experimented with different settings in /etc/exports on the client and in using the mount command only. Here's what I have in /etc/exports:
wogglebug:/mnt/HD_a1/threshNet /tnet-backup nfs rsize=8192,wsize=8192,timeo=14,intr 0 0
(Uh, the system names on my LAN are all from the Oz books -- at the request of a very young relative.)
When I try mounting it, I get:
[root@scarecrow:~]$ mount /tnet-backup
mount: wogglebug:/mnt/HD_a1/threshNet failed, reason given by server: Permission denied
I've also tried:
mount -t nfs wogglebug:/mnt/HD_a1/threshNet /tnet-backup
I've compared notes over and over, so by now I'm wondering if it's a typo somewhere (which is hard for me to spot, due to a learning disability). I spent 75 minutes online with tech support and that was unsuccessful. I am currently using an SMB mount to transfer files to the DNS-321, but I need NFS so I can mount it on my systems and use rsync to do backups that include the file owner and other info that rsync can't modify through SMB.
I've seen threads with answers elsewhere and I think I'm doing what they suggest. I copied settings from several answers r!ng0 gave, but they were pretty much what I was doing anyway.
I'm at the point of adding funplug, but I really don't have more time to spend on this and I really want this as just a black box that stores files through NFS.
Any help is appreciated!