From a bluetooth point of view every carrier is apparently different or at least potentially different. See all the separate OS updaters at:
http://www.palm.com/us/support...dater/
My Verizon Treo 650 does connect with the M series but treats the car as though it were a giant passive bluetooth ear piece i.e. the car can receive calls but not make them. (My Treo treats my Treo-brand ear piece the same way.) A work-around is to hit the Phone button which forces a reconnection, but the Treo disconnects in about 10 seconds unless a call command has been completed in that time.
When I called Verizon customer support six or so months ago, they told me that Verizon does not support Treo 650 bluetooth connections to cars. The end. No further comment. Thanks for using Verizon.
But that is a peculiarity of Verizon Treo 650's. At least some other carriers have different bluetooth modules in their OS package and apparently don't have this problem. Reportedly Verizon Treo 700's don't have the problem either. One hacker on this forum reported he was able to use a different carrier's bluetooth module in his Verizon phone and it all worked. Why Verizon hasn't fixed it is beyond me - but I did downrate Verizon to last place in a quality survey because of it.