Mac and NFS don't play nice together. I spent days trying to get this to work between my Arch Linux box and my Mac. After much effort and much research on the web, and many failed alternatives, I gave up.
I have settled on good old ssh - it is more powerful than you may think. I don't know if Ubuntu supports it, but look into sshfs, a virtual file system based on ssh. If you have sshfs loaded on your Linux box, you can specify your Mac in /etc/fstab as being of type "sshfs" and it works like a charm. I have been very pleased with it.
Also, if you are running KDE, you can use Konqueror's fish:// protocol to connect up to the ssh server on your Mac. In Konqueror's address line, just put in "fish://ip-address-of-your-mac" and it will happily connect. I do this frequently from my SuSE Linux install, which I haven't loaded sshfs onto.
By the way, if you don't want to have to keep entering the IP address of your Mac all the time, and it is fairly static, you can add an entry to /etc/hosts with that IP address and any symbolic name that suits your fancy. Then you can specify a name instead of an IP address in sshfs and/or fish.
Good luck!!