After typing this up I did a search on Onstar privacy and came up with this article on how law enforcement can't use it to eavesdrop on conversations in the car. http://news.com.com/2100-1029_3-5109435.html
That article was a follow up to Onstar filling a protest lawsuit against the FBI. If you read this article @ http://news.com.com/2100-1029_3-5109435.html you can see how the FBI already exploited the service.