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.
All of the emissions inspection equipment I am writing software for now include a CAN interface. Hopefully going to a standardized communication interface/protocol will relieve the myriad connectivity problems OBDII suffered from with each manufacturer relying on their own protocol. But I doubt it. Most State I/M programs are have revised their regs to require CAM. For example the OBDII program starting in PA will requires CAM capable analyzers.