Sadly, Apple has never actually fixed this issue for the 3-4 years it's been known about. Apparently the screens do not use pure white LEDs, but rather a blue LED with a yellow phosphorous coating that, when combined, approximate white. I can only guess that these bulbs lay across the base of the screen, which may account for better whites towards the top of the display and yellowness near the bottom.
This panel is manufactured by LG, and I reciently picked up a smaller panel by them in the 22" range that was deemed a best buy. While it has a ever so slight red push (you can correct) I noticed it had superior uniformity of blacks (no bottom screen bleed during movies), and NO yellowing to the screen's whites. It too is a LED Display. While certainly lower resolution than the Thunderbolt display, I was rather shocked to see better color out of a $250 budget panel.
More expensive displays can correct for color anomolies and whatnot via a hardware solution built-in. For whatever reason, Apple knows about the problem but doesn't do this, and we as designers and other cretive pros who rely upon their expensive solutions get short changed. Before returning, try recalibrating the screen. I have found that this panel, used both in the Thunderbolt Display and iMac, needs to be set at a color temp of 85 to 90k in order to start to appear neutral. When calibrating, click through each advanced step without adjusting, and, on the last screen, set the color temperature and save. You'll see Apple's notice right there that 65k is the "normal" range for a display, yet this one needs almost 90k? Once you're using the highter k setting for a few minutes, switch back to the default iMac/Tbolt display setting. You'll see a massive amount of yellowing to the entire screen at this point.
Last fall I went through 5 iMacs because of screen yellow and black bar issues. I finally gave in, settled on one with the least defect, and called it a day. 9 months or so into having the machine, other hardware issues cropped up and blotches started to appear down near the Apple logo. I was pretty upset and rightly went back to the Apple store, told them my previous headaches in having to purchase, install on, wipe, and subsiquently return numourus machines. I demanded either a full refund or a straight-up exchange I could sell off. Know what they did? They disassembled the machine and replaced 50% of the parts. What if they had to do that with your new car, would you ever trust that company again to make a reliable product?
After getting the now refurbished iMac back, I saw that it was even more yellow than the previous panel. I had enough. Apple Canada called me up to find out how my repair was, and I told the rep my case. "Oh, sorry to hear that -- can you brin it back into the store so they can have another look at it?". I started laughing. These things way a ton and to constantly be bringing it back and fourth was also costly, but they don't care. So I just said "sure", and then turned around and sold it off at a loss. I think I lost close to $500 dollars thanks to Apple. Buyer beware.
