I rented a D800 a few weeks ago and absolutely loved it. My current camera is a D7000 and I shoot a lot of wildlife. I rarely go above ISO 800 with the D7K and I wanted to see how the D800 would work at higher ISO. All I can say is wow.
These photos were taken with my AFS 300mm f4 and a 1.7 TC, except the horse (no TC). I cropped about 25% off of the full FX images and resized. Edited in CNX. I had NR off in camera, so what you see is the minimum the camera applies above 1600.
The photo of the squirrel at ISO 6400 blew me away.
Click on the images to see larger photos in my album. EXIF included.