While driving one warm, sunny day, my iPhone 4 was sitting next to me on the car seat. There's nothing unusual about this except that it was in direct sunlight. Even though the inside of the car was cool, thanks to the air conditioner, it was too much for my poor iPhone which temporarily overheated.
While the phone was cooling down the only thing I could do with it was make an emergency phone call or take a screenshot. About 20 minutes after moving it out of direct sunlight it resumed normal operations and all was well.