I may have an adapter to sell soon if the new iPhone 3.0 firmware delivers on new bluetooth profiles (to enable audio streaming via bluetooth to the car stereo)
Sure, a bluetooth adapter ill be required for folks who don't have Bluetooth capable vehicles, but you can't ignore the convenience of cable-free audio through the car stereo.
Video of iPhone 3G v3.0 streaming Bluetooth audio:
http://www.iphonehacks.com/2009/04/v...bluetooth.html