iPhone stuck on vibrate

A few hours ago I plugged my iPhone into the AC adapter to charge, and a few minutes later heard what I thought was an incoming call. It turns out my iPhone is now stuck on vibrate. My 'enter passcode' screen is there but locked up and the swirling 'working' icon is doing it's thing while the iPhone just vibrates over and over. I tried holding down the home key, I tried holding down the on/off button, and I tried plugging it into my computer via the usb.
Has anyone seen this one before?
I suppose I'm just going to have to wait 'til the battery dies and it shuts itself down?

Any help would be appreciated.