ThinkGeek are selling this thing called the Pacemaker Handheld DJ System. It is mega expensive and you might was well get a Korg 10 for the DS or some proper DJ equipment for the $500 price tag, but I really liked the advert.
And then I seen this video below where someone has used a RF output device to listen to the Pacemaker's output on his car stereo system. Which is cool because that could be another evolution of in-car entertainment.
1) In car radio (music chosen for you)
2) In car Tape/CD player (the music you want to listen to)
4) In car MP3 player (on demand audio, music, mixes, podcasts etc)
5) In car DJ system/music game (on the fly music creation)