I have the same problem, phone calls and 3G use make the handset very warm.
Apple replaced my handset no questions asked on 23 December, and the replacement handset has begun doing the very same thing.
So will be getting back in touch with them in the next few days.
Can't offer a solution, but you're not alone.
On the positive side I currently have a pulled calf muscle, so after using the phone I can put it on my leg to relive the pain a little!!!