There are different bluetooth profiles. Even though you can connect to your car and have a conversation over bluetooth, you will not be able to listen to music this way. As Remington already pointed out, the OEM systems that come pre-installed in most cars don't support the
A2DP profile. My wife has an aftermarket bluetooth kit in her car, and she can stream music to it from my Pre. It's a really handy feature, and I hope we will eventually see it become standard in all car bluetooth systems.