You probably know already some 27-inch iMac encounter some display problems which are materialized by a sudden flicker of the screen. According to Apple this problem is purely software and there would be no problem with the graphics card or on the backlight of the screen.

Apple technicians have received an internal memo stating that an update addressing the issue would be available in the month of February. Until then, customers will surely do so with this problem. As explained above just put a screen saver by pressing CTRL + SHIFT + Eject and then reactivate the screen by pressing a button.