yeah, ive noticed this too. Especially around the black and dark areas of my DIVXs (blocky artifacts)
They run fine on the PC though (cant notice this at all)
i'm thinking a onscreen contrast control for XBMP might fix this though. I had the same prob in SplinterCell where the lights on his back were making "green transparent blocks" around him in the dark. Bumped up the contrast and it fixed it.