Use your headset and save battery!
Auto Switching Bluetooth BETA 1.05 is nice piece of free software that enables Bluetooh whenever you receive or make a phone call, allowing the device to spare precious battery time by not having Bluetooth always on.
This is still under development and works fine with incoming calls, it still needs some work though when it comes to outgoing calls.
Nevertheless its free and it helps you increase the battery lifetime, so get it over at Modaco.