I had issues with iPad 2 and dust under the screen (no, I don't live or work in a dusty environment). First one I got had it within a week, and it wasn't just one speck, it was 2 or 3. I traded for a second one that had what looked like white residue on the under side of the screen (looked like a water mark kind of). Traded that one for one with a big giant white piece of debris dead center of the screen (didn't see it at the Apple store somehow). My fourth one was mostly perfect. Still had a couple of small pieces under the screen, but nothing that I could see with the screen on. It also came preinstalled with a couple of minor scratches on the bezel, but I decided to live with it, as I was out of patience by then.
My new iPad is perfect so far. No dust, no dead pixels, no weird light problems, no blemishes. I think sometimes when you get refurbs, since they are units that have been taken apart and put back together, they just get dust in there during the process. That doesn't really explain how I got dust in my original unit, but that's all I can think of.
But my new motto is "don't inspect". It's easy to inspect the iPad while you're wiping the screen off. Resist the urge. It will drive you batty.