I am pretty certain it can survive for a long time. The thing that gets hot is the glass not the metal frame. And when it's hot it's not hot as in u touch and u r gonna burn yourself.
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Ahhh hell with it, I have just taken one on contract... If I will regret it - no more Sony's for me.
Anyway, the heat is the problem in the long term, mate. Excessive glass heat means that the glue may take off (I think the glass panel is glued into the frame) and/or the glass can crack (and often does, have a look at the thread on Sony's forum...) because of temperature variations. Plastic and aluminum will not crack but glass might.
Anyway, I hope I got a good one, 24 months contract is a long time to commit to a phone so I hope this one is worth it...