DPReview spent some time with the D800, and we took the Canon for a spin last week. We scaled the D800 sample down to 22.3 megapixels to match the 5D, then pasted a 300-by-400 1:1 pixel section from each camera side-by-side in the image above. The D800 JPEG (on the left) appears to be the noisier of the two, which seems logical, considering that Nikon opted to boost the camera's resolution instead of its sensitivity. Still, the cam's top-ISO is quite usable, and if you plan to shoot in a studio setting or can live without a six-digit sensitivity, the D800 will likely suit you just fine. Hit up our source link for samples shot at the full ISO range, including full-res downloads, to make that call for yourself.
Update: We've replaced the Canon sample with a studio shot from DPReview, which provides a more accurate comparison. You can find images from both cameras at our source links below.