I was able to take the D800 out on a shoot at 5am and 5°F yesterday morning. The lens movement got a bit sluggish but the camera handled the conditions great. Battery had 3/4s of a charge after shooting about 260 pictures over two hours.
Most of the shots were at 6400 ISO and came out great. Some of the shots like the one attached were tough going handheld and would have been better done on a tripod.
I did take the camera into the house to take pictures of the damage at the very end, so I was dodging water and insulation but again the camera held up great. I am not sure of the D800's waterproofness but it seems to hold up to cold, snow and light rain so far with a good thorough cleaning after being in those conditions.