About a month ago, I posted a thread speculating the cause of the yellow tinge problem and color/tint variability on the new iMac 21" and 27" models. Since then, I received my 27" iMac which had the problem, and a subsequent replacement which was much better but not perfect. After following the thread on the Apple discussion forums concerning this issue, I read this patent document posted by Jacques LaPorte for U.S. Patent #20090135583A1 (received by Apple on May 28 2009) which details "a display system with a distributed LED backlight." According to the patent description, this is a new display technology which includes a "plurality of tile LED sources." Figure 1 on page 2 shows that this panel is intended for desktop systems, i.e. the iMac. Figure 2 on the third page shows the exploded view of the display panel and complicated arrangement of various layers in the panel. Apple itself alludes to the difficulty in using LEDs in large panels (larger than 20 inches) such as those in the iMac:  Another challenge with utilizing LEDs in large arrays is maintaining uniformity of color in the large numbers of LEDs. ...  The color and output of each LED also depend fairly sensitively on temperature. The difficulties in providing proper thermal management capability can readily lead to temperature variations across a distributed array of LED light sources. Since the color qualities of LED light sources are sensitively dependent upon their operating temperatures, such non-uniformities lead to unacceptable variations in color from one portion of the display to another. Apple's patent design is an attempt to address these issues using an array of LED "tiles" to deal with thermal and structural concerns as well as individual LED variability. Perhaps some of the yellow screen problems are still due to thermal problems. Most interesting to me is the presence of multiple "backlight diffuser sheets" (item 212) which are between the LCD sub-assembly (208) and the reflector (214). As I posted in my previous thread, someone who wrote to Gizmodo claims that it is distortions in the backlight diffuser sheets which is causing the tint variability in the iMac panels. I have to say that this is the most plausible reason for the problem as I noticed in my own iMac what appears to be a "curtain effect" of alternating blue and yellow tint vertical columns that looks like it could be due to a diffuser material that has bunched up or irregular underneath the LCD panel. This may be due to the manufacturing process, dislocation during shipping or the inherent design of the panel. Which brings me to the bad news. Since this is Apple's patent and design, this is not a situation of a "few bad panels" from LG, but may be the basic nature of this display technology that would be present no matter who manufactured the part. Many on the forum have commented that the Dell 27" U2711 uses the same display. While the LCD sub-assembly itself might be shared with the Dell, they are not using Apple's patent design for the LED tile array and diffusers. Bottom line is that unless Apple were to radically redesign or switch technologies (highly unlikely until a major revision), I think that the color/tint variability issues may unfortunately be with us for the duration.