while I'm trying to share NFS directories between my linux box (Slackware 10.2) and my MacBook laptop (Mac OS X 10.4), I read some topics and documentation that could help you.
But while it's my first post here, I cannot attach any URL
But you can search for the NFS-HOWTO on google.
On LQ, you can watch the topic named "NFS: Ubuntu (server) and Mac OS X (clients)"
It seems that the UID of the client user must be the same as the UID of the owner of the exported directory.