Since there is already a thread on D3s vs 1DIV, lets start one against the 5D MK II. I'm very impressed with the Canon especially on a camera that's almost half the price of the D3s.
I personally don't give a damn because I'm not a pixel peeper and prefer 21.1 mp for post processing and cropability. ISO may not be as good, but detail is definitely better. Resize your 21.1 mp photos in photoshop to Nikon's 12mp output, you'll probably get similar usable images at high ISOs.
Does anyone also know the history why Nikon's only go down to ISO 200. If you're comparing lowest possible setting between cameras, shouldn't they be comparing 100 vs. 200 and so on going up the scale? Not sure what the ISO standards are and if a manufacturer can market it as 200 ISO but internally they process to 100. So unless there are guidelines and sensitivity standards, not sure if you can compare ISO 1600 on Canon vs 1600 on Nikon equally? Maybe 800 vs 1600 to be more fair?