I had some time play with a friend's iPhone 5 in the car today (2007 Nissan Altima) and the results were mixed. In short, the connect / disconnect / connect again / disconnect again problem the tcasady described, and that many have mentioned in the past, is gone. Calls both sent and received connected fine and held their connection. Alas, however, the phone will still not ring in the car. After 2 rings, the display on the radio will show the incoming number, but there will be no audible ring.
By way of caparison, I also set up a blackberry 9900 at the same time. Rings in car, no issue.
I use an iPhone 4, but still on iOS 5.x. When I get around to updating, I will compare my results w/ Brisco811's.
Thanks to all for continuing to report on this ever-so-frustrating issue.