Multiple tipsters, French site Nowhere Else, MacNN, and AppleInsider itself have all reported first-hand accounts of the back of white iPhone 3GS models turning brown or pink in certain areas that are usually towards the edges.
When the problem manifests itself can vary but usually follows sometime after particularly intensive activity that generates significant heat, such as using 3G data extensively, playing games or navigating with GPS. It's also suspected that engaging in regular tasks for a long time, such as playing music or browsing the web on Wi-Fi, can also trigger the symptoms.
Speculation exists that the issue centers 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 flawed battery cells that react badly to the added stress.
iPhone 3G models in the same color aren't affected by the same problem, nor do black iPhone 3GS models appear to suffer the issue at this stage — though these last owners aren't completely exempt from the root causes. A number of reports have surfaced that some phones are getting particularly hot under certain circumstances, such as using the handset while it's connected to a power source.
iPhone 3GS with discoloration on the left; iPhone 3G on the right.
Apple so far hasn't commented on the issue and has taken to closing a large discussion thread on its forum centering on the subject. However, some have had success obtaining replacements.