About a week ago I started noticing the screen brightness randomly changing on my mid-2011 27" i7 iMac. The screen is darkening the equivalent of one to two brightness settings from full brightness, which is typically where I have it set during the day. Sometimes it only flickers for a second or two, but often stays at the darker setting for several minutes. I do not have the ambient brightness change setting checked in the Display control panel. I'm running Snow Leopard 10.6.8 and have already performed a PRAM and SMC reset. I'd like to find out if this could potentially be a system/firmware issue, or if this points to a physical problem with the backlight, its power supply or the video card. I know that LED screens adjust their brightness with a modulator, but not sure if this would be located in the power supply or the screen itself. The iMac is still under warranty, but I'm loathe to have to haul it in to the Apple Store, particularly since this is an intermittent problem. Has anyone else had this issue?