After reading this thread this morning, I got a scare this afternoon while using my iPhone 4 that's on beta 4. I was taking a picture and all the sudden my screen when dark with just a spinning wheel in the middle (like what shows up for a second when the phone is shutting down). After about 20 seconds of this, my lock screen came up, like the phone had resprung or something. It was weird, and now I can't duplicate it.