so are you saying with this cable that will come out in the future it will work?
That's right. Previous HTC phones (Touch Pro and Touch Pro 2) have had an adapter available that plugged into the USB port on the phone and then allowed an A/V cable (for TV out), or just an audio cable plugged into the adapter, as well as another mini-usb port to plug in a charger.
I assume that because the Incredible has been advertised as having TV out, that a similar adapter will soon be available, and you will be able to leave the adapter plugged into a charger, as well as an audio (or a/v cable) plugged in to a car stereo w/ an aux input, and then merely plug the adapter into the phone when you get in the car.
as to your original question of being able to use the USB input of a car stereo, I think the only option there is to use the phone is storage mode, and access MP3's stored on the SD card (and that may be a problem because the incredible has 2 storage devices available via USB -maybe I'll try it tomorrow on my Kenwood w/ USB input and see!).