I'm sorry to say that I'm glad it's not just my phone. Lately mine has started overheating while in my pocket! I have a stock nexus s 4g from sprint, not rooted or anything. Been experiencing lag and crappy battery life since ICS. But the overheating thing is relatively new and making me crazy.
When I'm leaving the house, I make sure the battery is charged, kill all running apps-- using advanced task killer, lock the phone and then put it in my pocket.
By the time I get to school or work (~20 minutes), the phone is uncomfortably hot in my pocket and it's unresponsive. Can't unlock or reboot, always have to pull the battery and let it cool for 20 minutes. Once it's working again, the battery is down to ~30% charged!
Gotten to the point that I keep the phone turned off until I actually need it to call someone or check messages. Then turn it off again. So much for having a smartphone...can't even enjoy my apps or connectivity. Phone is only 1yr old.