Kinda hard to believe that completely.

1) We never heard about this issue with the 3G. If it was, it was at a much smaller scale.
2) Apple took the trouble of inserting a heat warning page, which is a clear indication they knew about this for the 3GS.
3) This is reported as personal experience by reporters of major tech publishers (not just some blogger somewhere). For example : http://www.pcworld.com/article/167495/apple_iphone_3gs_overheating_repor...