One of the hinges on my MacBook Air detached, and a crack appeared, all rather suddenly and with no apparent cause. When I took it to an Apple Store, I learned that because of the way the laptop is assembled, it would be necessary to replace the entire display to fix the problem. Initially, I was told that this costly repair would not be covered by the AppleCare extended warranty because it was judged accidental damage.
I investigated further and found out that other Airs have experienced the exact same hinge problem. Armed with this information, I went to another (smaller and less busy) Apple Store, where they immediately agreed to make the repair under AppleCare. My Air is now fixed, and I'm once again a happy Apple customer.
If you have an Air and it has broken in this way, let Apple know you're aware that this is a common problem and insist that it should be covered by warranty. And if you have an Air and it's still okay, consider not lifting and carrying it in one hand: someone suggests this is what leads to damage on the fragile hinge, and I think that's a reasonable theory.