I'd have to second the Mr Clean Magic Eraser. That thing will take off any kind of marker. On Cartridges, you have to carefull on the Labels, as it will take the color right off. For DVD Game cases and the actual cert portion of a game, it works great, just be sure to wet it some. It doesn't work well if its dry. On average, i can completely remove Sharpie from a DVD case with a Magic Eraser (slightly wetted) in about 5 swipes.
P.S> Works Great on older consoles too!!