I think the problem here is the use of the word "cheap", when really, I see this as a design flaw. I wouldn't consider a phone bending in your pocket user error. I quite often put phones in my back pocket, and yes, that includes the iPhone and have never seen damage remotely like this. This isn't something that, IMO, should happen without much more force exerted than a person could reasonable exert by simply sitting down, unless we are encroaching on 300lb+ people in every case which, while I can't rule out, seems improbable.
But did Apple cheap out? I think they just want with a very thin design. When you go thinner and lighter, you also lose structural support. It's sort of the name of the game.
People in this thread are comparing apples to oranges. AFAIK there is no polycarbonate body phone out there are thing as the iPhone 5 (yet). So it is pretty tough to say that polycarbonate is better than aluminum in this sense. I am going to go out on a limb and say that the polycarbonate, if made thin enough, is going to have similar limitations. Perhaps they will manifest more as cracks, rather than bends, but the end result will be the same; there will be structural damage to the phone.