Here is the screen that you might see on your iPhone 3G or 3GS, as Apple has released an official document on "Keeping iPhone 3G and iPhone 3GS within acceptable operating temperatures."
The warnings are all simple and reasonable. Apple reminds users that safe operational temperature is between 0º and 35º C and that the phone should be stored, off, in environment between -20º and 45º C. If you don't follow these guidelines, things can go wrong like battery life shortening, the display dimming, connection loss, etc etc etc.
So is Apple addressing a real overheating problem or are they simply publishing reasonable operational guidelines? One thing is for sure: Years after the original iPhone's release seems a bit late (and thereby a bit too coincidental) to start passing along pretty standard electronic operational guidelines. [Apple via GearLog]