I use my unlocked GSM Treo 650 with my E90 and it works wonderfully!!!
I followed some simple precautions I found on bimmerfest before I paired it for the first time:
1) Made sure my SpeedDial favorites do not have any extra numbers that are set to be dialed automatically
2) Deleted my SMS messages (I have no idea why this is required, but I didn't take any chances).
After pairing your phone, your favorites (the ones that are of SpeedDial type) are transferred to the E90. You can even dial them by voice command!
This said, I know that there are slightly varying versions of Treo Bluetooth out there, so what works for my phone won't necessarily work on a Sprint phone or a Verizon phone, etc. There is a large discussion on this on bimmerfest.
VERY satisfied Treo 650 and E90 user.