We don't know how many people have hit this problem (the Apple support forums talk about only overheating here and here, as a sample), but at least one user claims that his white iPhone 3GS has become PINK with heat.
Not that there's much to take away from this, but just take a look at your white iPhone 3GS. If it looks like the photos here, then you have a problem. Or you've been eating cotton candy and you need to wipe your hands first before checking your email. [Nowhereelse via Engadget]