"The best solution to get rid of the problem is to take the MacBook Air’s bottom left end, and the top right end (of the keyboard) and flex it out. Yes seriously, gentle flexing does straighten it out. By being careful and flexing it once or twice, I succeed at having the four rubbed pads laying flat on the table.
I suggest you to do it incrementally, with gentle flexing (toward the top or bottom), until your MacBook Air lays flat. So gentle flex, try to wobble it with the lid open and close, check if the lid is still closing perfectly (from every angle), and then flex it again (if needed) until you reach the perfect balance."
I found the solution here:
http://6ixpassions.com/post/3712000157/your-macbook-air-is-wobbling-on-a-flat-su rface-fix-it