Is it possible, yes
Is GM doing that, do not know the answer but it is known that GM's intent is for updates to be "passthrough" to the point when cars pass a dealership that car will be queried and updates done
Onstar it is quite possible GM can do the updates but I rather doubt it is being done at this time only in the fact if the update goes bad the car would be hosed and would have to be trucked to a dealer to replace the ECM.
This issue expands greatly, many services such as cops, states, etc see reasons they would want to control a car if its due to speeding, not paying your taxes or car payment, etc.
Some states have been putting transponders along the road way to do such spying and Ca has already passed a law saying the state has the right to monitor or control cars when need be via wireless.
