Apple has responded to reports of the iPhone getting hot:
Apple, however, has issued what some might call a "common sense" warning: a support document that warns users not to keep the iPhone in an environment where temperatures can exceed 113 degrees Fahrenheit, including parked cars.
If it overheats, you might see this:

I've had the iPhone get pretty hot when I've had multiple radios going (WiFi, 3G, and Bluetooth) - although the battery ran down before I ever got close to a warning like that.
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