I'm pretty sure that the GPS works when you call 911 because on most (if not all) phone models I've worked with, you have select 'location on' and if your location is off it's still on as '911'. I believe this is also why Analog and TDMA was turned off, they were untrackable.
And also, the point of OnStar is for the system to recognize when you've had an accident. I couldn't see OnStar going onto cell phones because thats part of their marketing is how a cell phone can fly out of your hand when you crash.