About the Yellow Tinge (Tint)
The yellow tinge is a hardware problem and is a function of the LCD screen and/or backlight (the new iMacs use a LG Electronics LED-backlit screen with a TFT active-matrix liquid crystal display utilizing IPS technology). It mainly affects the lower third of the screen so the problem is that an image could look warmer on the bottom (however the whole tint of the screen can be off as demonstrated below). The screen is pretty large so depending on your viewing angle the colors can shift a bit too. Not all screens show the issue to the same extent though (and depending on how you calibrate your monitor it can be more apparent... for example if you load different ColorSync profiles for the display it can become more obvious).To test your computer and see if it has the Yellow Tinge problem run the test program.
If you report the problem to Apple they will either offer replacement or repair. Some other good notes and photos are available at this MacRumors discussion thread.
Comparison Between Screens
There can be a fairly big difference in the color tinting between computers when you compare them side by side (suggesting that there are variances happening during manufacturing). When I compare 2 of my new iMacs the difference is easily noticeable (the screen of one is much more yellow looking and the bottom half is more yellow than the top). The photo to the right used the same lighting and camera settings. The Week 48 model (which I consider to be a "perfect" iMac) has a neutral looking screen whereas the Week 51 model is very yellow.
To have this much variation in screen color between "identical" computers raises doubts as to how useful these screens are for any serious graphic design or photography based work (not to mention the issue of varying color on the same screen). It's also likely that most people don't realize their screen color is yellowish as they don't have anything to compare it to. Tonal differences between screens should not be this extreme and color consistency within the same screen should be much more accurate.