I thought I had noticed this just after the update. Mine is a week 46 and has been fine since purchase and to be honest is fine now.
One thing I have noticed - I opened Automator and watched the quicktime video which, by the way was terrible to listen too. However, the colour a light grey background when watched flickers very gradually(bottom right of the window) - this might be my eyes? but I have found the screen to be less responsive.
Can someone try the video and let me know if they see the same?
This thread is being locked as it is getting very long and causing some browsers to time out. The thread is continued in [New iMac 27inch screen flickering/tearing/shutoff (continued)|http://discussions.apple.com/thread.jspa?threadID=2330035].