It looks like in Snow Leopard (OS X 10.6), Apple broke some of the smartiness in NFS mounting from Finder. In 10.5, you could just slap nfs://host/path into Finder's "Connect to Server" dialog and it would happily mount just fine. In 10.6, it doesn't and here's what you get...

On the server, a mountd success message is the only indication that it received communication:

Oct  5 09:15:59 nfsserver mountd[5551]: authenticated mount request from 1.2.3.4:1015 for /nfsshare (/nfsshare)

On the client, the dialog box will briefly show the progress bar and then revert back to the "Connect to Server" input box. When mounting on the command-line, you'll get:

bash-3.2# mount_nfs nfsserver:/nfsshare /nfsmount
mount_nfs: /nfsmount: Operation not permitted

The disconnect is that Linux by default expects "secure" ports to be used for NFS. Mac OS X on the other hand defaults now to "insecure" ports. The secret sauce to get it to mount via mount_nfs is to specify the resvport mount option on the command-line or the better option of telling Linux to allow insecure ports for NFS communication in /etc/exports. Doing the latter will allow you to mount via the "Connect to Server" dialog or on the command-line.

If you just want to mount a Linux NFS mount (that by default does not allow "insecure" ports), use this on the Mac OS X command-line:

mount_nfs -o resvport nfsserver:/nfsshare /nfsmount

If you want to allow Mac OS X to mount Linux NFS mounts on the server, make sure your export line contains insecure. Here's an example line in /etc/exports:

/nfsshare          1.2.3.5(ro,async,no_root_squash,no_subtree_check,insecure)

Linux enforces an idealistic multi-user machine view of the old world. This concept of defining "insecure" and "secure" based on privileged port numbers is antiquated and doesn't apply in a world where everyone has ultimate privileges on their desktop machines.