After seeing a post on dmctalk.com (cant find it anymore) yesterday I got an idea for a winter project. The idea is this
[*] Buy a single-din folding touch screen with or without the actual head unit
[*] Buy lowpower/lowheat Atom based XP computer with a 12v -> 5v power converter to feed it. These things come in such small form factors these days you can stick it anywhere.
[*] Alternatively get a gumstix pc with Windows XP Embedded (http://www.gumstix.com/3rdparty-winCE.html)
[*] Attach an small SSD or 2.5" removable HD for audio/video
[*] Attach an amplifier to the PC for sound
[*] Develop a custom interface using Microsoft.Net/WPF for an Awesome custom MP3/Video player
All this hardware does add up a bit, but I can develop the software no problem (that is what I do for a living), and I suppose I can wire everything together. You'd miss the radio and the cd-player of course. (Might even throw in some bluetooth connectivity).
It sounds like a hell of a job, but oh my what a nice touch it would be to have a completely custom stereo.