Apple iPhone iOS 5.1 - Manual (Page 178)

iPhone operating temperature
If the interior temperature of iPhone exceeds normal operating temperatures, you may experience
the following as it attempts to regulate its temperature:
iPhone stops charging. Â
The screen dims. Â
The cellular signal is weak. Â
A temperature warning screen appears. Â
Important: You can't use iPhone while the temperature warning screen is displayed, except to
make an emergency call. If iPhone can’t regulate its internal temperature, it goes into deep sleep
mode until it cools. You can't make an emergency call when iPhone is in this mode. Move iPhone
to a cooler location and wait a few minutes before trying to use iPhone again.
178 Appendix B Support and Other Information