Sharing Music with the iPhone

There's a cool little feature I just discovered thanks to a friend at work. If you are on a phone call, and turn on your iPod music, everyone on the call can hear your music! Also, the volume controls for the phone call and the music are separate so you can have some nice background music to your call while still being able to hear soft voices loud and clear. I can't decide if this is a designed in feature or a happy side-effect of something else, but I think it's cool.