Not sure if my solution addresses the problems described here - but it might help. I just got a new car (Subaru Forester) and started using bluetooth to connect to my Droid X. Every time I got in the car, my phone started automatically playing music - not over the car sound system, just on the phone. It was using Pandora, so on Pandora I went to "preferences" and selected "off" for Bluetooth for Automotive. Great! Problem solved.
The next time I got in my car, the phone started playing music using the Android music app that came on the phone. It doesn't seem to have an Options or Preferences settings, so the Pandora solution wouldn't work. Found the answer in the Droid X users guide - in app settings, go to "Wireless & Networks", choose Bluetooth settings, then press and hold the name of the bluetooth device - a popup opens; choose options, then you see "Profiles". There are two settings - "Phone" and "Media" which can be checked off or on to provide audio over the bluetooth. Uncheck the "Use for Media audio".
This won't work if you ever want to use the phone to play music over the bluetooth, but I intend to use my ipod for music, so it works for me.