I've seen a few topics on this in the past, but I just noticed mine has this issue as well. My left front feet is slightly higher than the rest about a couple millimeters or so and causes my mba to rock a bit when I place my wrists on that corner. I've tested this on several surfaces and determined that for the most part, I've seen the same symptoms. Any ideas if this is an actual defect on the bottom plate due to warpage or if it's a common problem that is caused by uneven tightening of the screws? I'm trying to decide whether I should buy a pentalobe to figure this out since I don't really have the time/luxury to get to the closest Apple Store...