Onstar sent out an email shortly after they made the announcement about the tracking. They have revised their position on it and will completely disable the Onstar feature from your car when you cancel. The only way it can be returned to service is if you push the button. Even if your car falls off the face of the earth - unless you push the button you'll be on your own.
They don't track your movements. However, they do track how you drive and vehicle speed. The only thing they do with the information is for GM research, unless law enforcement requests the information then it can be turned over to them with proper procedures being followed like responding to a court order.