A Rio Riot or Apple iPod ...

And let's not forget the Creative Nomad Jukebox III... it has some very nice features...

Out of curiousity... would it be feasible to mount a Rio Central in the trunk and extend the LCD (using LVDS or something similar) and IR remote sensor to the dash of a vehicle? I imagine (having never used the central) that it would make one kick-a$$ (albeit pricey) car audio system.