AppleTV is technically supported, but there’s no apps yet
And someone from Tuaw blog, named Erica Sadun, tried its luck to jailbreak the Apple Tv and she succeed to do so:
The Limera1n page has updated to indicate that Apple TV jb is "technically supported". I can confirm that I put my ATV2 into DFU mode and was able to "jailbreak" it. But since no AFC services or OpenSSH were installed (let alone APT), there's nothing that really can be done with it at this time.