Mute the external microphone when a headphone is plugged in?

I'm running Gingerbread on my Samsung Galaxy S II. I dock it on my dash in my car and plug a 3.5mm cord from the phone to my stereo aux input. I often listen to music from my phone through my car stereo, but when a call comes in and I answer it, there is terrible feedback on the other end because the caller hears both my speakers and the audio from the phone. Is there anything I can do about this other than to switch to a traditional headset?