"I see the D3x as the clear winner. It's clean and sharp. The fur looks like fur and has real texture everywhere, as well as being clean. No noise and real texture means high signal to noise ratio, and the winner!"
Ken Rockwell is a goof. While I found the crop comparisons pretty insightful, his conclusion that the D3 "trashes" the 5D is a little hasty. I would say that the D3 has a marginal edge straight out of the camera, because it seems to use more chroma noise reduction than the 5D. But chroma noise is very easy to deal with in RAW, and considering that the D3 seems to preserve the same amount of fine detail, I would say that the cameras are about equal. There's obviously more luminance noise reduction going on in the D3's default setting as well, but I think it's handled FAR more judiciously than with the D300. I would also like to point out the different points of focus in the D3 crop and the 5D crop directly below it: the 5D is focused more towards the very front of the bear, towards his eye, and the D3 is obviously more focused to the back, where the beige fur meets the brown. I believe that their different points of focus hinder, but don't prevent, a comparison of fine detail. Considering this, I don't see any profound difference between the two cameras in their ability to capture fine detail.
I do, however, think Nikon's efforts to catch up with Canon (Full frame only) is an impressive feat, but these crops don't provide enough evidence to say that Nikon's D3 trashes the 5D. I think it's equally impressive that the older 5D is still able to compete with a newer camera with respect to fine detail at high ISO. I believe that Nikon's next generation of CMOS cameras will be more impressive than this first generation.
FYI, I don't own either camera.