I started a thread like this a while back, it was the battery. A few people have had this problem. Hopefully my store will be getting the replacement for mine tomorrow (they had to bring in a red phone that I found several hours away). If they don't get it, the phone is going back tomorrow and I just wont have a working phone for a few days... thanksgiving just had to be 13 days after launch day.
@robomonkey: Mine does this when I gently tilt my phone. I began shaking it to see why my phone felt funny when changing orientations. Feels a little bit like haptic feedback. And when you tip the phone when recording a video, you get a horrible CLUNK.