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.
I think Apple should buy up Nintendo (I don't know what their cap is but assuming it is doable). They could even try to just enter the console gaming market themselves with the iGame or something similar...
Kind of fun to think about what an Apple-made car would be like.