According to French blog Nowhere Else, a white iPhone 3GS, while under a lot of CPU load, left faded brown streaks on the back plastic cover. According to the site, the user said:
I did the test several apps warning radars in the car when I was a passenger, the apps using the GPS chip as well as 3G, after a while my iPhone was very hot but not burning hot.
The user apparently called Apple customer service who wanted the iPhone to do some "analysis".
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