Do you need to advertise an AFP service via Avahi for an Ubuntu Server to show up in OSX Finder?
According to this Mac OS X Hints posting, browsing for NFS broke/was removed in OS X 10.5, and I can't find any indication that it's ever come back. The post links to a script for automounting Bonjour-discovered NFS shares on OS X, and it looks like there's a newer version on github under the name bonjour-nfs.