I've experienced bluetooth problems. In my case I have significant signal degradation in the pairing with my car. I'm using a 3Gs upgraded to iOS4. Prior to iOS4 I had no issues. So same phone, same car, same mount position in the car but degraded bluetooth signal with iOS4. Everything points to a problem with the Bluetooth stack in iOS 4 on the 3Gs and hopefully this will be addressed with iOS 4 update(upgrade guide in ifunia iphone column).