Mine has gotten REALLY hot when in certain games which was a little alarming at first. However I then quit the game and the temperature seemed to drop very fast, which suggests the heat dissipation of the chip is extremely good so unlikely to be a problem.
I would say just keep your hand away from the metal around the camera as its obviously being used as a heatsink for the SoC. My only worry is that the heat might damage the camera, but surely they thought of that?
The ONLY time I can see it really being a problem is when using the phone. The glass and metal earpiece area gets hot too, would be pretty uncomfortable on your ear. I haven't had it get very hot watching videos or YouTube though, certainly not like how it did just now in a game.