I just called Apple Care because I have the same issue with my VW Bluetooth Kit (Jetta 2011) and because this car model does not have a Bluetooth button that let you answer your incoming calls through the bluetooth system you have to answer them through the phone itself. There's no way to route the incoming call to the car sterereo automatically.
Not even can listen the GPS (telenav plus for iphone) if the Bluetooth is turned on!!
This is because the new Bluethoot technology that IOS 5 is using (Bluetooth 4 wireless technology)
Very annoying by the way.
With my old and outdated BB I did not have any problem so far, with the "Most amazing iPhone yet" I do !!