Link We are as confused as you. Why does the ATI chipset force dithering on 8 bit displays? Unknown. Why does the Macbook output force dithering on 8 bit displays? Unknown.
We are pretty confused by it, but it's pretty obvious to see. Get REALLY close to a solid color image (like the grey desktop background) on an OSX system, on ANY display, and you'll see the pixels shimmering. It's 100% on all the time and it's inexplicable.