Whether or not reports of iPhones overheating are true, Apple has taken them seriously enough to reveal the presence of a temperature warning screen for the iPhone 3G and 3GS.
Apple, have issued a support document that advises users not to keep the iPhone in an environment where temperatures can exceed 113 degrees Fahrenheit, including parked cars.
Using the iPhone in temperatures over 95 degrees can also trigger the temperature warning. Apple warns that Low- or high-temperature conditions might temporarily shorten battery life or cause the device to temporarily stop working properly. That will prompt the device to trigger the temperature warning above, essentially locking it except for emergency calls.
If the phone exceeds temperatures of 95 degrees, the iPhone may stop charging, its display might dim, a weak signal may be experienced, and the temperature warning screen above may also appear. Apple’s support document implies that there’s a temperature sensor of some sort built in to the iPhone 3G or 3G S. If that happens, , you need to let the iPhone cool down before you use it again, although the phone may be able to place emergency calls during that time.
But Apple also warns that CPU-intensive applications, such as playing music or using the GPS while in direct sunlight may also overheat the iPhone.