After comparing the sample photos available so far the D800 seem to be the better camera for me producing images with details quite comparable with MF backs just a year ago. This is especially true for the D800E. It is somewhat understandable because of the much higher resolution. At the same time the noise characteristics - or the in camera NR - is also better on the Nikon. Either this or for some unfathomable reason Canon did not post the best photos that can be created using it.
BTW on the other hand the ergonomics of the Canon body is much better IMO.
I can afford to be unbiased I think because both cameras are out of reach for me at the moment. And not because they're yet unavailable