First generation OnStar units used a 5 watt AMPS phone with a simple 103 AFSK modem and a GPS. The car could be called and a command sent to retrieve the GPS or unlock the doors. I haven't seen the hardware for the current OnStar devices, but I'm certain it includes a new CDMA cellular phone, an in-band modem whose supplier shall remain nameless and a GPS. In addition, the new OnStar gear has access to the vehicle networks to permit door unlock and query of the diagnostic modules (similar to what a service shop can do). Remote engine diagnostics capability is interesting to fleet owners. Ditto for the theft recovery. Most people buy OnStar for roadside assistance. Well if the first generation used an AMPS cellphone then you could listen to it with you modified scanner. It was hardly secure and wouldn't require the goverment to erect their own "cell" to listen to your communications. I am sure the latest revision intergrates with the vehicles OBDII compliant computer. Combine that with a GPS and they know everything about you and you car. When and where you were going how fast you were driving. How often you stopped to get gas. If you overrevved your engine etc. I work in the vehicle Diagnostics industry but before that I was a "hacker"
The OnStar GPS doesn't track you in real time. You must make a voice connection to the car and transmit a query command via the inband modem to query the location. I'll bet the OBDII queries are limited to a few specific diagnostic locations. The bandwidth on OBDII just isn't high enough to monitor a wide variety of items in an operating vehicle without impacting the performance of the necessary message passing.
Have you seen any indication of a wholesale switch to CAN bus? Many of the manufacturers that my group has been assisting over the past couple years seem to be switching to CAN in favor of the multiple variants of J1850.