High ISO is harder to call. My feeling is that it will be about a stop poorer than the Canons (although not all the Canon's are the same). These are very varied. Some even report that the Nikon is very good!
"I was less impressed with high ISO test files that I saw which were taken by a colleague. A seat-of-the-pants judgment is that ISO 3200 with the D2x looks like ISO 1600 on the Canon 1Ds MKII, and ISO 1600 on the Nikon is not as good as ISO 800 on the Canon. Let's call it a 1 stop disadvantage for the Nikon at high ISO. Low ISO images though are exceptional. I don't know yet about mid-range speeds, such as ISO 400, and we'll have to await knowledge tester's results before any firm overall conclusions can be drawn."
Michael H. Reichmann (Canon user)
http://www.luminous-landscape.com/
"ISO sensitivity comparison:
ISO100-200 : No obvious difference
ISO400 : 1DsMk2 starts showing some noise in the shade under chin. D2X has no problem.
ISO800 : D2X starts showing some noise in bright gray area. 1DsMk2 looks the same as ISO400.
ISO1600 : D2X has less noise than 1DsMk2."
Cameraman Magazine, Japan. Sourced from DPreview.
"Some german guys who have been presented the D2x during a "Nikon professional user day" last week were all very very excited with the quality, they also praised the ISO performance."
DPreview. Unreliable!
"Most importantly for me, is the fact that I was shooting at ISO 800 and at that speed the quality of the image was exceptional. To give you some kind of comparison. I have been using the D2H now for a long time and as we all know there is some noise with those cameras at higher ISO values.
However, with the D2H there is very little noise from ISO 200 to ISO 400. The image quality of the D2X at 800 ISO is very similar to what you would expect to see from the D2H between ISO 200 and 400. The image quality is very very very good and that was at the 7 megapixel image size as well. The color quality and the skin tones are exceptional."
Rick Rickman (Nikon User)
http://www.rickmanphoto.com/about_txt.html
"Now to the high ISO stuff. After reading the notice on MR's site, I was expecting some pretty bad stuff. However, I didn't find that to be the case. The camera did have a tendency to underexpose some of the shots (likely thrown off by some bright areas in the images --- nothing new here), but the noise levels at 800 were quite good.
Nikon has done quite a bit of in-camera noise processing on this camera. They have done a VERY good job controlling the chroma noise. There is next to no random color noise -- unlike any of the current canon cameras. However, this does not come without a price. Just like if you remove the chroma noise using software, you'll get some artifacts. In the bokeh, there can be some odd splotchy areas of color that don't look very natural. I've gotten the same thing when using chroma reduction in post production. It may look a little odd in print, and I hope there is SOME way to control the in camrea chroma reduction. If not though, it's not a huge deal.
Since there is some pretty strong chroma reduction, the images take on a more film-like look. There is a bit of a grit to them, but nothing out of the ordinary for the modern crop of digital SLR cameras. I don't have a direct comparision available, but I would venture to guess that straight from the camera, the D2X may have a little more noise at ISO 800 than the 1 series cameras from canon, but the images will likely print nicer, and more film like."
Ben Horne (Canon 1Ds Mk2 shooter)
http://www.benhorne.com/
I should also add that Michael H. Reichmann (Canon user)
http://www.luminous-landscape.com/ posted a comparison test on high ISO noise between the D2X and 20D. This test generated a huge amount of web banter. The test showed the 20D to be considerably superior but he withdrew it because "enough errors in my methodology were pointed out to me that I've decided to withdraw the piece until a more rigorous and comprehensive test can be undertaken. I regret any inconvenience that this premature publication may have caused. "