I just finished reading a review which includes some info from Canon's Chuck Westfall. Chuck says that the sensor is a 2 stop improvement, and its not just improved NR processing in camera, but a two stop better sensor.http://www.robgalbraith.com/bins/content_page.asp?cid=7-11675-12364
I find that amazing, but from my playing with the full size jpegs downloaded from DPR, it is possible, I want to see the raws first. I printed 8.5 X 11 images on my Epson 3880 this morning of the images at ISO 12,800, 25600, 52,200, and 102400. The images were clear and sharp with no visible noiise, unless, of course, you used magnification. I did not think it was possible, so I reprinted them carefully and the results were the same. Anyone would be happy to have a 8 X 10 print at ISO 102800 if this can be reproduced in the real world of images. At any rate, i think ISO 25600 is going to be useful.
The Images from the D800 that I've seen indicate about 1 or 1.5 stops less, which is also supurb, particularly considering the high MP count. We really need high ISO images downsized to 22mp to compare.