Update: I picked up Imac number 3 this weekend (4th screen in total) and this one appears to be clean. I don't have the build date info in front of me but I will post that as well. I'm going to let it settle in a for at least a few more days before I pass final judgement, but it appears to be a good machine from every perspective. Note that the screen is not perfect, but the yellowing on this one seems extrememly limited to the point where there are no real discernable "areas" that draw my attention to them. The last one that went back was disticntly yellowed on the bottom 20% of the display (my first two screens were absolutely miserable). The new one currently shows very little, if any, yellow tinge unless you run the test pattern and even then it seems negligible.
I will commend Apple tech support on this. I had opened a case a little while back just so that there was a history on it, and told them I would wait until a resolution was found. When I called to check status, I was immediately passed to a higher level engineer who asked a lot of questions and said that they wanted the machine back in their possession for testing (but it was ultimately my choice...). I had puchased it the week before Christmas, and he still really wanted me to exchange it for a new machine, which he facillitated through my local Apple Store. The engineer was extrememly helpful, very focused on my user experience, and at times, applogetic (after he ran through all of my cases from every iMac.screen I went through over the last couple months...).
Note that at no point did he admit outright that there was a known problem; only that my experience was unacceptable in Apple's eyes and that their only concern was making me a happy customer. This is important, because I haven't seen any evidence that Apple has officialy acknowledged a specific issue. That's a little disappointing, because you could tell by the questions he asked and the tone of his voice that this is not new to them - they have seen it before. Still, it was top notch support (as it has always been whenever I call) and I was generally impressed with how things were handled.
As mentioned, I don't feel like I'm out of the woods yet; I still want to give this new iMac a few more days of typical use before I pass judgement, but as long as nothing crops up, life should be good. The machine has been a bit of a disappointment, but Apple Support has been excellent.
As an asside, Time Machine actually works really well. I had my old machine config back in about an hour, and there was a LOT of stuff on there. Had to reinstall iTunes by hand for some odd reason, but that was trivial.
