I'm on a 2010 13" MBP, and displaying certain darker shades of gray will result in fairly noticeable flickering, at least when the shade of gray is covering the whole screen. This is especially apparent at the highest brightness setting. I know that there are certain patterns that can result in flickering, but I wasn't aware that the same was true for solid colors?! Specifically, I can provoke the flickering by displaying a full-screen solid color at a brightness of 21% on the gray scale... The effect is not noticeable at this size, so you'll have to click on the image, for the 1280x800 version, or use Preview.app to scale it up to even higher resolutions. You'll probably want to raise the brightness to the highest level as well.