Pictures? While no LCD is perfect, a certain amount of unevenness is normal. I'd have to see a picture. Chances are, if you notice it, it's a problem. I don't notice any anomalies on my display.
I've gone through 3 Air's and all had some sort of that... All about the same.
As the darkened spots in the corners are xtremely obvious & visible, you think that it warrants for an exchange then?
I think only you can determine that. If you feel that you're seeing it all the time in ordinary usage, I'd say yes, go try to exchange it. If it's something that you notice only very occasionally, then I'd try to accept it. But you paid all that money, so you should demand what you feel makes you happy. I'm inclined to say that you should get it replaced, out of principle.
Good luck!