Official AppleTV MacCar Thread
There is another thread on the AppleTV, but the OP there was asking specifically about which head unit to use and that got hijacked. I thought it would be a good idea to have a dedicated AppleTV thread to discuss the pros, cons and viability of using the AppleTV in the car.
Some pros are that the ATV is smaller and cheaper than a Mac Mini.
One of the negatives to using the AppleTV is the power supply.
I made an inquiry with Carnetix and Mike H. over there said, "Unfortunately, from what I understand this thing has a built-in AC power supply. Perhaps someone can open one up to see if the AC supply can be removed/bypassed."
Another negative is the lower RAM and processor power. The way I see it, we will need to run an enhanced version of the AppleTV OS that can run more codecs, USB turned on, ability to use Bluetooth, etc... OR and pared down version of Mac OSX so it isn't overtaxing the guts of the machine rendering it useless.
I was really hoping that the AppleTV would be great for the car, but the more I look at it, it looks like there might be more effort to get it going, than is worth it.
Here are some good resources on hacking the AppleTV:
http://www.appletvhacks.net (This site appears to be down as of 4/2/07 12:20pm MST)