I've tested two apps that use the BT mic.
Super BT Froyo. Basically, all audio from the phone goes to your headset and all sound from your headset goes to your phone. Goes back to normal on phone calls. Have to turn it off before turning off headset or phone resets.
Ear Buddy also sends voice and audio via BT mono but only sends music when music is playing, not all the time, and allows you to initiate google voice search by pressing the bluetooth button on your headset!
Haven't tested Ear Buddy fully yet, but there's an update that fixed a major phone reset problem it was having.