Didn't Some one say....
It's all about the platform.
Seriously though, you would need to get the motor, smart phone and auto accessory industries to agree about standard interfaces, which isn't just putting USB into a car. You would also need to ensure that the user's device choices were safe in a driving environment. Checking out your music library whilst driving isn't safe. Watching a movie on your smart phone isn't safe, and so on.
My current Golf likes iPhones and has simple controls that allow you to use it with the inbuilt CD/Radio and Phone system without taking your hands of the steering wheel. The phone is locked away in a special compartment, so you can't fiddle with it whilst driving. This is sensible, but you can't use anything other than an Apple iPhone or iPod. The car also locks out a lot of it's own configuration functions when the vehicle is moving, so extending that to selecting songs would be a good move.
I doubt it would be too difficult to do this for all smart phones. Anything more sophisticated would be distracting and dangerous.