Chuck Westfall has already stated that the high ISO improvements are two stops in the sensor and not in the firmware NR. In the 1D MK IV, for example, he gave credit to about 1/2 stop over the 1D MK III for beinng sensor improvements. And, the 1Dx is supposed to be even better!
I downloaded all the full resolution test images from DPR, and was simply amazed. When I pulled up the blacks as far as LR4 would go, there was no banding. Then, I printed them at 8.5 X 11, and was even more amazed, all the prints were clean and sharp with noise seen only when looking very closely.
Now, I'd like to see RAW images.