I'm looking for the exact same thing. Too bad the BMW BT doesn't support a2dp. The iPhone should be able to do 2 BT profiles at once. I currently have my BT earpiece and the BMW paired with my phone. The iPhone let's me select which device I want it to link with. I would assume that we could pair the iPhone with the A2DP receiver (if we could find such an animal) as well as the car at the same time. If a call comes in, we may have to select the BMW handsfree from the list... not sure if the phone is smart enough (yet) to pair a default device for A2DP and another default device for phone calls. If you find the adapter you are looking for, please send me a PM or an email.
They claim it'll mute audio when a call is answered, and the call will be routed to the 3.5mm jack instead. Whether the little mic in the body of the thing is any good, I have no idea.
There's also a little remote that may or may not be of any use, and a port on the side that'll take the iPhone's USB cable so you can charge if need be.
I got the missus an iPhone 3GS a couple of weeks ago, and the car kit just arrived today. Here's hoping it all works and that they've done their homework and not created any ground loops or anything like that - I'll chime in once I've had a chance to get everything set up.