Originally Posted by Deception
Could be a bad sync with the music... Try restoring and re-syncing, that might fix your issue.
Uh ... no. There is no possibility that this is a sync issue.
OP doesn't tell us which head unit he has. I'm guessing it's a 730N, as I've seen some reports of this behavior. The fix seems to be to make sure you have current iOS and to have your phone running the music player ap, not showing the home screen.
If you have a 430N, be sure you've had TSB 08-036-11 rev A applied. It affects iPhone behavior with USB, not Bluetooth, but it's worth having. My iPod didn't work before this TSB.