I'll answer because no one's bit yet. I'm not certain, but I doubt it. Streaming stereo music requires a different Bluetooth profile, and automotive BT implementations are notoriously finicky and restrictive (see the
Bluetooth forum). In fact, the Treo 650, while able to be coaxed into cooperating with Bimmer's BT system to make a call, can't even download your contacts into the car's directory like many other BT phones can (Palm's fault, not BMW's). I know because I tried it in a 2006 535i.
But hey, if you can afford a Treo and a BMW, you can afford an iPod. These are lovely gadgets, specific for music, and, thanks to an Apple/BMW partnership, will completely integrate into your car's stereo system. A new 2GB Nano will set you back all of $150, and holds enough music to drive from NY to Miami and not hear the same song twice.
-K