Via NYT > Technology:
Some Apple iPads are overheating in the sun, although its unclear if this is a widespread problem or confined to a small percentage of iPads.
This happened to me a few times when I was sitting outside in the sun with my iPad. It will suddenly shut down and show a temperature warning screen.
Yup, this happens to me, too. I had it happen on my iPhone once or twice, but it is pretty common on my iPad if I leave it in the car. I think it's due to having the larger screen collect more sunlight.