Another thought: the standard slot for car stereos (DIN) is 2" tall by 7" wide, with varying front to back room, but usually at least around 7". The Mini Mac is 2" tall by 6.5" wide, by 6.5" deep. If someone came out with a touch screen LCD “radio face plate” sized screen that plugged into the standard VGA port and figured out the power issues, you’d have yourself one hell of a car stereo.
Put WiFi in the Mini Mac and you could keep all your audio synced with your car while you slept. Hook up a GPS and you’ve got a navigation system. Add Bluetooth and you can piggyback on your mobile phone’s data connection. Turn on OS X’s built in voice recognition and tell your stereo what to do.
You could really do some cool stuff.