I read this thread, when experiencing the same problem with a 2 day old, MacBook Air 13".
Rather than guess, I took my machine to the Apple Store.
The kind of screen hinge problem listed in the initial question here is not normal. The 'Genius' replaced my 2 day old MacBook Air with a new one, which has a much better, stiffer screen hinge. I can now open the lid to any angle and move the MacBook Air around, without the lid moving.
If your MacBook Air is within warrenty, don't settle for a hardware defect. Get it replaced or repaired and enjoy your Mac again. We don't buy Apple hardware to have to compromise on quality.
I hope my experience helps someone.