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):

Page 1

http://ascii.jp/elem/000/000/024/24797/index.html

Page 2

http://ascii.jp/elem/000/000/024/24797/index-2.html

Page 3

http://ascii.jp/elem/000/000/024/24797/index-3.html

perhaps, it is easier if you use such serives as babelfish to translate it.

http://babelfish.altavista.com/