Another Apple TV owner got creative and published a video showing Super Mario Brothers game running on the Apple TV.
Hacking the Apple TV became pretty popular in a very short amount of time. There are hardware hacks available from adding a larger hard-drive or make the Apple TV work on 480i TVs. Software hacks enable you to play DivX and now to play old Nintendo games.
See the video on Gizmodo. See also Gizmodo's Top 5 Apple TV Hacks.