I did this a while back so i could actually watch live tv (using a EyeTV DTT tuner) on my apple tv, and it works very nicely

however my only gripe with it is the lack of ram, i did look at upgradeing it, and although i could find pin compatible ram chips that would give it 512mb or 1gb of ram, apparently there is a eeprom chip that would need hacking, which i would have no idea how to go about doing

But apart from that it made a nice low power media player without the restrictions of the atv os