Mystery vibrating notifications?

I like notifications for the iPhone in iOS5, but my phone keeps vibrating once in a while and I can't figure out why!

I don't believe that I'm accidentally hitting the mute switch because it happens when the phone is just laying on a table.

The problem is that my phone will vibrate, similar to getting a text message, but nothing shows up on the screen. I've looked in Settings > Notifications and can't seem to find the culprit. Any idea what it could be? I've turned off a lot of apps that receive notifications, but I still can't figure out what's causing it.

Is anyone else getting this behavior? Is this a bug of some kind? I have a new iPhone 4S.