Reports are hitting the Internet about the iPhone 3GS and overheating issues. Some are even speculating that a recall could be necessary. Apple has not responded to these claims at this time.
An informal poll around the office reveals that none of the 3GS owners at G4 are experiencing any overheating problems. Have you experienced anything like this? What about with different iPhone models? Past experiences lead me to believe that most overheating issues are actually caused by software.
If you are experiencing any issues, try a software reset and wipe the phone. It just might solve your issue.