I noticed while back when I was reading up on the GT2, the Porsche Navi has a built in TV turner. Going by thier website, it can pick up digital signals. The word (on this site) has been that no one makes digital tv turners yet for cars, but there must be some way.
Follow the link a read the PCM feature. Yes, I know its Porsche and for a 200k car, it better have a tv turner, but I thought it was interesting none the less...