Focus Numerique has posted ISO samples of the new Canon EOS 5D Mark III and they show a lighter AA filter, resulting in slightly sharper images overall, and cleaner high-ISO images.
In general, the difference is not large. I would say less than half a stop. But that only tells half the story. The Mark II appears better at extracting high-ISO detail in textures, shadow noise is much lower, and roll off into highlights is better. If the camera cost the same as the old Mark II, I'd call it a resounding success. At a price much more than the Mark II's launch, I still think that it is a worthy camera, but offers little to justify its enormous premium over its older sibling.
Obviously, I'll wait to see what the DxOMark is, because as with Sony's sensors, major advances in dynamic range and color are not always apparent in the final product. The Mark III may yet end up being a fantastic successor to the Mark II.