Basically, what happens is that over time and normal, non-abusive use the hinges on many MacBook Airs will loosen until one day the hinge catches and cracks.
It seems that Apple doesn’t like fixing these hinges unless you pay a meager $800.00, even when the computer is still on warranty. They call it “accidental” damage.
I will tell you this, I have never dropped or mishandled my MacBook Air and the hinges have been a little wobbly for the last few months. If they cracks I will politely make the good folks at Apple completely miserable if they do not fix them. The MacBook Air is a premium product and I expect them to stand behind it.
If an Apple Genius* tells you that you need to pay $800.00 to fix the hinges on your MacBook Air tell them, “No thank you, I think I will go out and buy a netbook for half that.” There is no plane of existence where that price is anything short of insulting.
Not everything Apple makes is perfect and they need to do right by their customers.