It's been said that Apple TV is running on Mac OS X.
A Japanese web news service, ascii.jp/mac took out the internal HD from AppleTV connect it to a Mac and found it is actually running on "AppleTV OS v10.4.7"; the version number suggest that it is a variation of Mac OS X "Tiger"
The article also tries to boot Mac wth that OS and vice versa but both attempts end in vain.
You can read the article here (in Japanese):
perhaps, it is easier if you use such serives as babelfish to translate it.