Felt the need to chime in, because I had this problem with my iPhone 4 and my iPad 2. (People will say I'm lying about all this, but I assure you I'm not.)
iPhone 4:
#1 Got a dead pixel
#2 Had a pretty noticable blemish on the antenna band
#3 The one I finally lived with -- has a small nick by the bottom screws, but it doesn't bug me.
iPad 2:
#1 Tiny scratch on the bezel (not something I would return for), but it also got a bunch of dust under the screen about 2 weeks into owning it.
#2 No cosmetic problems, but developed 3 dead pixels all toward the center of the screen.
#3 HUGE piece of dust dead center of the screen and visible when the screen was on.
#4 Was told this was my last exchange--got it home and it had some small specks of dust under the screen--nothing real noticeable. But it had MAJOR scratches on the bezel. Learned to live with it.
When I got my new iPad, I vowed not to inspect it too closely, and break this cycle of returning for minor imperfections. I have noticed a very minor mark on the bezel and one on the Apple logo--only visible in very specific kinds of direct lighting. There is one dead pixel toward the edge of the screen, but I can only see it when I look up close without my contacts in. With them in, my close up vision isn't even good enough to focus on it. Most of the time I can't even remember where to find it and don't see it during normal use.
I have made the decision that stuff like this isn't worth all the time and energy it takes from your life to exchange over and over. It's up to you how much the dead pixels or dirt bug you, and don't think I'm telling you not to return. BUT....
No iPad you get is going to be perfect, I guarantee it. Give me 10 people who say they got a perfect iPad, and I bet I can find flaws on at least 8 of them. I am VERY nitpicky about stuff like that. And the problem is that you will keep taking these things back, keep finding things wrong with them, and you will just get more and more frustrated. I've lived it, and I just hate the fact that I wasted energy worrying about it. For this iPad, it's going to be the first Apple product I've ever bought that I have decided to not sweat the small stuff. It's a mass produced gadget, and while Apple has conditioned us to appreciate the beauty of their products, these are not precious gems we're dealing with here. They're just gadgets.