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Although I'm not on the O2 network; I'm in the States on AT&T. I have the same problem, my iPhone gets hot at the top on the back side of the phone. I doesn't matter if I'm on EDGE or WiFi, if I'm using safari then it gets hot.

And I thought was just my iPhone, I'm glad to see that I'm not alone.
 
Looks like a normal behavior. That's where the CPU and WiFi chipsets are.
 
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