Thanks for this, was looking for the cause over and over – seems a Quicktime issue. QT X and QT 7 show heavy banding, with MPlayer X everything looks fine, like in the Motion viewer.
I use one (Asus GTX680 2GB) in a custom built mac, no problems with FCP X nor Motion. Slice X works as well. It seems faster than my GTX285 on my old MacPro 2008.