Engadget has a post about i3's application that is supposed to make it easier to use a Mac in your car. It is pretty much a launcher that can launch any other applications. I think that this is a start, but is not the ideal. Just launching iTunes is not ideal, especially since the iTunes interface is not great for a car application.
Since most of the applications on the mac (iTunes, DVD player) are Applescriptable, it should be possible to create an application that has a simplistic interface like i3, but actually controls the applications themselves. For example, this application could present an iPod-like interface, which actually controls iTunes. Or this application could display a list of movies, like Matinee, and launch VLC or DVD player to do the playing