For what reason if you don't mind my asking? Dead pixels?
And which model is that?
Dust and dirt have crept in between the glass and screen and there is strange black/light black discoloration along the borders of the screen on all sides that is shaped sort of like a crescent moon on each edge. One area looks like there is a handprint inside now with 4 finger tips. It isn't really but it looks like that.
So basically, a year and half later it looks like hell. This of course did not happen overnight. It's been getting progressively worse for months from when I first noticed it. I guess the screen was never properly sealed but I also wonder if it isn't otherwise defective given the curious distortions around the edges.
This is a mid-2011 27" iMac that has this problem.
Oh and I forgot to mention that just the other day I noticed artifacting when moving the mouse around on the screen but I haven't seen it since. This was not long after a few gaming sessions which resulted in the GPU overheating and shutting itself off. I forget which game now which is really bad since it was very recently. I think it might have been Crysis Warhead played in bootcamp that was doing this but then magically stopped doing it without any intervention from me.
My guess seeing that behavior is that my GPU's days may be numbered also. We shall see. The GPU is the Radeon 6970m with 1 GIG RAM.
I notice when running Windows 7 in bootcamp that once the fans spin up, they go full out and stay that way even after exiting the game and time has passed. It seems as if they do not function correctly when Windows is running on this machine. However, if I play say World of Warcraft in OS X (on ultra settings at 1440p) and they spin up, they go back down soon after I exit. I understand this could vary by how demanding a particular game is but the behavior is really consistent in Windows. My iMac is loud whenever I play games in Windows. It isn't when I play games in OS X. I got this uncomfortable feeling like I was abusing my computer by playing games with bootcamp and since i hated rebooting anyway, I deleted it. My fans have not spun up hard since.
I love my iMac overall but as you can see above it isn't perfect, not this particular one anyway. So it's a good thing I got AppleCare so I wouldn't have to worry about it for at least three years. I figure if a GPU is going to fail with regular gaming, it will do it within three years. The screen issues I was not expecting but having heard about them being an issue on some iMacs I am not all that surprised either. I keep putting off taking this in for repairs because I hate to be without it but I really should soon. I paid damned good money for a high quality display and this ain't it.