I have the exact same problem running iOS 4.2.1 on iPhone 4 32GB. It seems to happen after a long period on standby, overnight for example, as I haven't seen it happen during the day while using it. This morning, mine was shut off. When I restarted, the battery life was still at 78%. I didn't have any apps running as far as I could tell. I've already had me phone replaced for SIM card and camera problems; not looking forward to another replacement.