I'd just be happy with a design built around a portable player. Get in the car and dock the portable player fully inside the dash. The DIN sized unit would have a seperate screen/interface for the car that pulls the music, playlists, and position info off the portable player.

Space and price depending, maybe also throw a hard drive inside the dash unit for either standalone use when the portable isn't around, or an expanded music library when the portable is docked. A really cool possibility of this would be to have compressed music on the portable player, but FLAC or higher bitrate compressed music on the built in drive. Dock the portable and it keeps playing, but using the higher quality music on the dash unit.

All a dream of course. The closest I think we will get is seeing someone do this around an iPod. Some of the interfaces are getting close, but not quite fully there.