This is a screenshot of the message I saw after picking up my iPhone 4 to take a photo. The iPhone had been lying idle for at least an hour before being picked up. No calls had come in. Nothing except normal background processing for things like email had been going on as far as I am aware. The air temperature is about 82F: Warm but not hot. So, why this message?
I should also note that I was not trying to use the flash.
I think I may need to make a visit to the local Apple Store Genius Bar soon.