Why would the thing flex? Would heat be a factor?
(Aside from the nasty dent I have no issues with mine)
It doesn't so much flex as deform over time in everyday use.
The shell is aluminium. As I said in your thread, it's a 'dead' metal, not a springy one like steel. So once it's shape is changed, it has a habit of staying changed. The Air does have a thick shell - indeed, it's necessary since it effectively has no internal chassis - but it can deform.
For example, if like me you carry it in a non-rigid messenger bag most of the time than it is subject to mild flexing forces, which can over time deform a machine built like the Air (or the Macbook Pro). I doubt from the relative ease that I can bend the Air that the metal is treated in a way that optimises Aluminium's strengths. It's made for relative ease of manufacture - trading off weight, form and rigidity.