OK guys, more interesting info:
I have a Tracfone (a Samsung T301G) which is a simple slider phone that has Bluetooth. I successfully paired the phone to my 2007 Sentra SL, and made and received phone calls with it. No dropped bluetooth connections, and everything works as intended.
However, my iPhone 4 with iOS 5.0.1 still has problems. I can pair it fine, but I will lose the bluetooth connection occasionally, and the iPhone will constantly try to reconnect with my Sentra. When it does work, I can make phone calls with the iPhone using the onboard Sentra bluetooth system, but incoming calls will not ring.
These problems just don't exist with the cheap Samsung slider phone in the same car.
The iPhone has been disconnected completely from the Sentra Bluetooth. What I'm doing now is using the Samsung as strictly my car phone, and enabled call forwarding on the iPhone to the Samsung (works beautifully.)