Yesterday I shot a full wedding with the new Nikon D4 that LensProToGo was kind enough to send me since mine isn't here yet. I was extremely pleased with the way that the camera performed and although I haven't gone through the images yet, I picked out 6 random shots that were taken at high ISO so that you guys can see the noise in real world situations.
Both of my shots taken at 12,800 ISO are still under-lit and they have motion blur meaning that these are very poor examples of the cameras quality at this setting. In my opinion ISO 12,800 is totally usable and I will go even higher the next chance that I get.
We should have a full review of this camera compared to the rest of Nikon's fleet in the next few days.