The iPhone 3G S does everything. It's a cell phone. It connects to the Internet! It's a compass and a GPS! And it can also function as a tiny space heater.

Almost since the release of the 3G S, reports have come in of them overheating and turning brown. Brown? Yes, brown.

According to Apple's knowledgebase, the problem stems from using resource-intensive programs for extended periods in direct sunlight and doesn't affect all phones.

I can't wait until the next model of non-vampire iPhones come out.

Keeping iPhone 3G and iPhone 3GS within acceptable operating temperatures [Consumer Reports Electronics]
Keeping iPhone 3G and iPhone 3GS within acceptable operating temperatures [Apple Support]
IPhone 3GS Handsets Overheat, Turn Brown [Wired]