Luckily, Sprint offers a "no questions asked" return policy if the device is returned within 14 days of purchase. Today, I returned the 650 to Sprint. I am sad to see it go back, but it is cheaper than selling my 2005 330xi or 2004 X5 4.4!!
I have a Sony Ericsson T637 that works without problems in both vehicles; although, the phone book does not transfer with the X5. My primary vehicle, the 2005 330xi, works great with the T637 phone. The phone book loads into the car, the steering wheel controls for the phone work great and the clarity of the conversations is very good.
What I lose is the great e-mail and PDA functionality of th Treo 650. I am convinced the problem is with BMW and not with the Treo. BMW was very smug with their useless response to my message sent to them via Owner's Circle. They tried to tell me the Treo was not on the compatibility list and then rattled off some nonsense about how they adhere to all these BT standards... If they cared about standards, I would not be writing this message!!
My next attempt at this will be when Cingular releases the GSM version of the Treo 650. I am not foolish enough to think it will work. But, I am foolish enough to give it a try.

Also, I hear the Blackberry 7100g will soon be released on Cingular's network. If this is true, I'll give that a try, too.
Good luck everyone!