What that guy did was still using the mass storage so he was basically just playing the contents of the microSD card via the car stereo's USB port.
It would be sweet to connect the Droid via USB like dropping an iPod in a dock and then output the audio via USB, whether it was an mp3, Pandora, or whatever.
I suspect, though, that to do that will be quite difficult. The dock would have to establish and ADB connection with the phone (and the phone would have to be placed in USB debugging mode), and even then, I'm not positive it would work. I think this will have to be something that Google will have to add to the ASOP project officially in order to get 3rd party dock support.