Feed my AppleTv with media from my Ubuntu desktop PC
Solution 1:
One possibility would perhaps be to run iTunes on the Ubuntu PC through Wine. From what I've heard, the syncing part doesn't work, but the airplay features does.
Second option is using Ubuntu One, if you only want to play audio as per : The Ubuntu One Blog
Third solution would be to install a media server on the Ubuntu PC as this guide dictates.
Disclaimer: Haven't got an Apple TV or even a Mac myself, so I haven't tested any of the proposed solutions. =)
Happy testing!