Every time I get a new mobile I plan to make some custom car mount with charging and audio outputs. Never got around to doing it though. Maybe if I get a Note it'll be worth doing with it's huge screen and I doubt it'll fit too well where I put my mobile now (on the dash next to the speedo).
I did once make a car computer though. With a beige desktop tower. And an inveter and keyboard. There was no screen so controlling it (only used for music) was a pain so it didn't stay in there long. Also it was a little awkward for people sitting in the back of the car with their feet resting on it :)