After issuing an advisory on how to prevent iPhone 3G and 3GS overheating earlier this month, Apple has now apparently found a potential cause of iPhone 3GS discoloration. Contrary to user reports suggesting these two issues were related, the company claims a likely cause for discoloration comes from people putting the iPhone inside a protective case - a common enough accessory. The assertion is that certain cases discolor when exposed to heat, not the iPhone. Though Apple hasn't made an official public statement regarding this, the report comes from dealing with Apple's technical support.
While Apple's explanation is certainly plausible, it does seem they are trying very hard to place blame anywhere other than on themselves. After all, if previous generation iPhones had no such issues, why would the 3GS be any different? The company has made many design changes with the 3GS, and though this situation may've not been predictable, Apple cannot say they played no part in it.
They did offer one suggestion for fixing the problem: cleaning the back of the device with rubbing alcohol. This seems to validate that the device itself is in no danger of becoming discolored from heat, with the real source of the problem coming from the outside world.