I guess we're going to have to agree to disagree. I have 2 late model 13' MBA'S and they BOTH do this. You have to understand, these are not "rigid" cases like the MBP, which I also have 2 of. Apple designed these computers to be light and ultra portable. I'm also going to disagree with you on the display models because every 13" display model at the Apple store the bottom would also flex when squeezed when picking it up. In my opinion the noise people are hearing when squeezing the bottom is from the battery being pushed up against the track pad. Lastly, if you did return your MBA and the one's on display didn't flex as you say, why didn't you return it for a different one?
Odd...........