The car stereos that come with USB ports can take in the wireless Internet USB sticks. In theory you should be able to add a chip inside the stereo that links to online services like iTunes to download music while driving.
A better option would be for car stereo makers to develop an operating system that can incorporate any 'music dialer' that connects to particular music site(s). That could be just the start. On top of this car stereo OS, you could then unleash the developer program and let people develop applications on top of this ‘platform’.
If you think about how much time you spend with the car stereo, the usage time would surely exceed your talk time on your mobile. Surely this ‘platform’ could be better leveraged.
The type of applications you could develop on top of the plain music listening: you could have a karaoke application that plays AC/DC if you want to let it out on your way back home from work; you could have a free music streaming service that is ad supported and perhaps asks you to answer a few survey questions each month to better target the audio ads; you could bundle the GPS navigation on the same intelligent stereo.
If the stereo has more than one USB port (and a mic) you can insert in the USB phone sticks of companies like Outsmart and GoIP and use the stereo as hands-free mobile phone. Or just plug in the USB phones provided by PC-to-Phone providers. So I guess there are lots of possibilities.