After months of living in denial about what many called a serious flaw with the MacBook Air's design, Apple has finally come to their senses with the release of a recent support document. http://support.apple.com/kb/TS2948
The support document details symptoms that include: lid not closing completely, broken or cracked plastic near one or both hinges, more than an inch of free play when opening or closing the lid, or the lid falls freely closed from a 30-degree open position.
If your MacBook Air has any of these symptoms you should bring it to an Apple-Authorized Service Provider - even if it is out of warranty and even if you've already paid for these repairs since you might be eligible for a refund.