How do I answer a phone call and use car speakers with Ford SYNC under iOS 5?
I had this same issue. Here is what I did to fix it.

Remove the bluetooth device from the Ford Sync. Dont just turn it off, actually delete the device from the car

Remove the Sync device from your iPhone. Again, delete the entry from your phone.

REBOOT THE iPHONE (don't forget to do this). 

Resync your phone with your car