Apple has issued a link in responce to widely reported claims of the iPhone overheating to the point that it can discolour the plastic housing. Optimistically they have titled it
"Keeping iPhone 3G and iPhone 3GS within acceptable operating temperatures"
Among the snippets of advice offered to prevent "exceeding the acceptable operating temperature" are -
not using certain applications in hot conditions or direct sunlight for long periods of time, such as GPS tracking in a car on a sunny day or listening to music while in direct sunlight.
Who would ever want to do that???
Pricelessπ