from the video's that i've seen of the ATV sunning OSX, it seems like the OS runs pretty smoothly. I have not seen any clips yet of video file playing or iTunes working or anything like that.
I hitnk the main drawback is the fact that the power supply is built into the system (from what I've heard)
This link has quite a few good pictures of the insides of the ATV. http://www.ipoding.com/modules.php?s...view_album.php
From thelooks of it, the power supply is on the inside, but not built into the main board. Knowing, this, there may be a power managemnet workaround that would be made to get the an ATV running OSX to wake and sleep on car on/off