This happened to me as well. You can simply un-pair the phone and re-pair it and it will work until you restart the vehicle. Then you will have to repeat the annoying procedure every time you start the vehicle.
Its frustrating because it used to work just fine (the only complaint was that it took a good 60 seconds or so until my nav even recognized the phone, thus leaving the bluetooth non functional until it was recognized) until I updated my phones firmware to the gingerbread operating system. Ever since I did that, my phone has been extremely slow to respond to simple menu browsing.
I'm not a certified technician but I think the correlation is that. I just hope Samsung updates this snail of a phone. Can't wait for the Samsung Galaxy S II to come out. I'm jumping ship as soon as its available.