This link contains an interesting discussion of a custom designed computer system tailored for car use, using a Baby AT motherboard, and a 6" Sharp LCD screen.
Sun also has an interesting article about a Java-enabled electric vehicle, entitled The Network Is The Car
Ryan's Car MP3 page has good photo documentation of a car based MP3 computer, along with comprehensive technical information. Worth a look.
Empeg Prototype page A page by the creator of the Empeg car MP3 player. Great technical info and advice. If you're looking to design a car based MP3 system, read this page.
Another MP3 car projectThis one is less elaborate than Ryan's Car MP3 page, but still worth a look.
More Car MP3 resources Brief info on LCD/keypad design pertinent to Car MP3 systems.
I remain in awe. This car looks like the Bat Mobile... complete with semi-automatic paintball gun and minature missiles.
Project Lorelei A more modest version of the above link.. still impressive though.
Another Car MP3 page, with a fair amount of tech info.
Sproggy's CarMP3 page I bumped into this fellow on IRC recently. He was very knowledgeable, and his site is worth checking out, because of the sheer technical detail and design notes inherent in it.
MP3Car A big, really impressive site on the topic of MP3Cars.

BTW, the reason I'm so interested in this topic is because I'm planning on doing something similar myself, with my Black 1990 Nissan Maxima. :)