See also Jan 25 entry and more on 5D Mark II. Just added to my DAP noise study is a field shot comparison between the Nikon D3x and Canon 5D Mark II, featuring three A/B crops as well as a pair of very large crops.
The results are unequivocal, and even more compelling when viewing a larger area of the image. Mouse over the image below to see the 5D Mark II result.
ISO 100: Nikon D3x