Car stereo/phone interaction

With the new features of car stereo's/dvd players and whatever else, I would like to be able to plug my phone into the stereo, and then have the stereo lower the volume of the music that I am blaring and alert me of the call, since my music always overpowers the sound of the cell phone ring. Ideally, the end user would be able to determine if they wanted the music lowered, or some other method of notification (whether it be a beeping, or a flashing light, or whatever) that is least annoying to them. Regardless of the method of notification, if the user answers the call, the music would lower to a user defined preset level that allows them to talk/hear the conversation. This could be combined with existing hands free technology (routing the audio through the speakers) or with regular phone usage, again user determined. The obvious problem here would arise from different phones and different makers of car stereo's, and the need for them to interact with one another, but ultimately those are things that could be overcome.