Apple has recently issued what some call a "common sense advisory" warning iPhone 3G and 3GS owners not to use there iPhones in temperatures exceeding 35º C (95º F).
If your iPhone does begin to exceed the maximum operating temperature then you may experience issues with network signal, display brightness, device not charging or a message will display (pictured) warning you.
If an overheating message is displayed, turn off the iPhone and allow it to cool. Whilst the message is display Apple states that emergency calls will most likely be able to be placed.