5.1.1 bluetooth problem

Something that seems to happen often with new firmware updates. Sadly, it is nothing Apple can fix since they don't test each and every bluetooth capable device in the world for compatibility, that would be insane.

You can try deleting the stereo from the BT phone settings and delete the phone from the car settings and retry the pairing process.

This article has helped people with your issue in previous threads.

http://support.apple.com/kb/TS3581