Here at CES 2012, OnStar has just announced their own closed API that will allow developers to tap into OnStar data for use in their own apps. They've also made it known that its first partner on the API front will be RelayRides, a social network that allows anyone to rent out their own vehicle when they aren't in use. This means that you can log in to the RelayRides app, and it will utilize OnStar services to find which cars are available, where they are, and will then allow you to book the car, and even unlock the doors and start it up, all using the smartphones app. Vehicle owners will be able to set their rates and times.
This is just the beginning of the potential of what we should expect from developers as it pertains to OnStar data. More partners will be announced soon, and developers are encouraged to sign up for access, which you can do by emailing firstname.lastname@example.org.