If anyone would like to see a "real world" example of the D800 at high ISO, I did a little experiment tonight.
I grabbed a common cereal box and photographed it. With a 5:4 crop in the camera. I used the fine print on the side that has the nutritional value. I then pulled the RAW image into LR4, cleaned it as I would any ISO6400 image. Then exported it at full resolution.
I also went back and did a 10% crop of that image, and exported it as well.
This image was shot handheld, with a Nikon 70-200/2.8 lens with VR. It was shot at 1/60 of a second. Much like anyone might do if they were shooting at an ISO this high.
The test is not meant to be scientific. It is designed to be illustrative of just what a user might get if they had to shoot the D800 at high ISO, and then crop the image. A common scenario for sports or wildlife shooters.