GM has way too many brands and selection for Apple. Apple could just design and build their own "Apple iCar" -- the only car with one button and a touch screen to control everything. They can sell it outside of select Apple retail stores.
However, the ZipCar, the New Beetle, the Mini Cooper, the Honda Element -- these are all increasingly appliance-like vehicles. I think Apple's take on a consumer appliance car would be interesting, especially if they could get their web services in order.
Kind of fun to think about what an Apple-made car would be like.