AIVMS does what you need with its bluetooth override feature. If bluetooth override is selected, when the headset attaches and changes the output line AIVMS automatically switches it back. It works well for headsets attaching to widcomm based bluetooth dongles, and we are adding BlueSoleil support soon.
Vehicle based VOIP communication is a primary focus with AIVMS, and an inexpensive bluetooth headset will function as a hands free voice over internet phone when AIVMS is running.