Now on my 9th one and about to return that as well. Blemishes under the glass on this one AND number 8...easily demonstrated to the folks in the store....sigh...it's becoming a personal crusade at this point.
If indeed these have all been genuine issue's and I'm not saying they weren't, then I think I would be giving up I think. I had to swap my first one for a tiny dust partical under the screen which was visible on white screens, but decided when I got my second that if it had a issue that warranted it being returned, that I just would'nt bother, and just get a refund and just continue to use our iPad1, at least for the time being.
Fortunately this 2nd one seems okay, i can spot a tiny amount of light bleed if I'm being really picky in one tiny area when viewed in a very dark room, and even then I have to look very close to see it, so in normal use and lighting conditions it's a none issue... so I'd rate it as being 99.5% perfect, I suspect if I wanted 100% perfection I may be opening several boxes, and even then being a picky person myself I would later find something I wasn't 100% happy with.
AIS I would have gave up after just 2 and doubt I'd have tried for a third time, I can only assume your much keener than I in wanting a iPad3. One would have thought the store staff would have gone over the replacement very carefully before leaving the store, though I acknowledge, that most times minor issues only become apparent to the user over the following few days. I sincerely hope you manage to get a 100% good one, preferably before Apple go bust...lol One thing I think we can conclude from this , is that Apple need to get their QC sorted, though in fairness, it is to be expected that new models will have an initial high return rate until production methods are optimum.
Best of luck with your next one.