I posted something on this a while back, and someone in the comments posted a great howto. I felt it is worthy of another link so it is easy to find. This basically lets your Linux box share files using the AFP protocol which is very fast and the native file sharing protocol for Mac OS X.

http://www.kremalicious.com/2008/06/ubuntu-as-mac-file-server-and-time-machine-volume/

This person built a DEB package for modified netatalk which works on Ubuntu Ibex:

http://gpz500.wordpress.com/2008/09/27/lairone-al-servizio-del-leopardo/