I want to add to this discussion since I am in the middle of this unfortunate iPad lottery as well.
I purchased an iPad 4 from a third-party vendor and the uneven backlighting issue was evident, with a slightly yellowish-gray cast in the upper left corner as well. To the average person it may not matter. But I use the iPad for creating content (art, sketches, etc.) and I am an avid reader (iBooks, internet, etc.) so this issue makes for a less than smooth viewing experience after long periods of time. On a blank white screen, it is evident that the center is the brightest, coolest area, while the lesser brightness frames the screen vertically on the right and left side (which seem to be the common characteristic issue I have been reading online). Also, if you create or find a neutral gray box or page and zoom in to cover the entire screen, you can also see any discoloration and uneveness as well.
I returned that one, and got a replacement from Apple directly online. This one doesn't have the yellowish-gray cast on the upper corner, but has the same vertical framing issue. I thought I would be used to it, but after a long period of looking at the screen during normal use (reading, content creation, etc.) it becomes obvious. Even more evident in lower lighting environments. Not everyone lives in a brightly lit envrionment like the Apple Store where flaws are diminished at first glance.
My plan has an endgame though. I'm going back to Apple Store, explain the issue and have blank pages ready to show. If the replacement exhibits the same characteristics, I will swap it for the refurbished iPad 3 that my father owns (he is okay with this). His has very good backlighting eveness and a slightly warmer cast (which is better than a bluish cast for long-term reading). My use of the iPad requires more visual attentiveness than his use, so I will give him the iPad 4 which for him, is mostly used for music, movies and videos.
Too bad that it has come to this, but as a 21-year user/owner of Apple products, who has an acutely trained eye for digital color due to my career, I have never had to play this game with any of their screen-based products (iPhone, iMacs, Displays) right out of the box.