I have also experienced this problem with several iMacs for years and posted in various threads here and on the Apple board. In my case, the blotches were caused by heat (white plastic iMac) once, and dust twice (aluminum iMacs). I fixed two of them myself since they were out of warranty anyway.
I recently took my last iMac to the Genius Bar. I didn't want to fix it myself this time since that particular machine was still under warranty. The Genius plugged it in, started it up, took one look at it and immediately saw the blotches. From his reaction, I would say he was familiar with this issue. He apologized and insisted that this was the first time he ever saw something like this. At that point, I actually explained to him what causes the blotches and how you can fix the issue by "simply" opening the panel and cleaning the diffuser with a cloth. He seemed completely puzzled the whole time. Hard to say if he was just acting. In any case, he explained that Apple would replace the screen for free since the iMac was still under warranty. If it happens again, I would have to pay for the next replacement since Apple believes that issues like this are usually caused by environmental conditions. For the record, this iMac was kept in a very clean office environment and never used for gaming. There is another all-in-one computer in the same room, plus another monitor. All of them have been there longer than this iMac, and none of the screens show a similar problem.
I realize there are many many happy iMac customers out there, but for me, this was the last iMac ever. Sure, you can fix the blotches yourself, but this should not be happening in the first place, and I have no intention of taking apart my iMac every couple of months. More than half of the iMacs our household owned over the past 6 years eventually developed the blotches, and we also had one yellow screen and two power supply unit failures. They look nice and perform well, but this is just too much.
