iPhone hangs up when headphones unplug

Before one of the updates (I think 3.0) I could listen to the iPod feature in the car via cassette adaptor, if someone called, I could just plug it, take the call, then plug back in without missing more than a beat or 2. However, on my new, updated 3.0.1 3g iphone, whenever I unplug it, it immediately hangs up on the caller. Is there some hidden option to turn this off? its pretty stinkin' annoying to have to call someone back EVERY single time it happens.