Some owners of Apple’s latest iPhone 3GS have been surprised just days after launch as models with white backs have been changing colors after frequent use and excessive heat. It appears the issue is focused on the battery: Rapid Repair’s Aaron Vronko, as well as those affected by the issue directly, claim the color shifts occur around the outline of the battery pack and that it’s likely due to goofy battery cells that react badly to the added stress.
[source - Appleinsider]