Had to take in my iMac last week(2010 27" i5) due to the infamous screen smudging issue. The top left side of the screen looked like someone touched the screen behind the glass with dirty fingers. It spread halfway down the screen in on the left side and was looking pretty bad before I took her in. Apple thankfully is taking care of it. They said the display was going out and needed to be replaced. Free of charge under AppleCare. If you notice this on your iMac, have it checked before AppleCare runs out! Otherwise its $625 to fix.. She is going to be hitting Craigslist when she comes back from repair though. I hear the problem is due to heat on the iMac's and that the issue will return more sooner than later.