Dpreview has released an in-depth review of the Canon EOS-1D Mark IV:

“From the point-of-view of the tasks it was built to tackle, there is nothing that can touch the detailed, high resolution images that it can deliver ten times a second.”

“Its high ISO performance is also very good indeed – it may not quite be able to match Nikon’s full-frame D3s in the very lowest of light but nothing can.”

DxO labs have posted an interested comparison between Canons flagship and the Nikon D3s stating that the Nikon camera has an incredible f-stop advantage of up to 1.7:

“The Nikon D3s achieves an exceptional result with a score of 3,253 and takes the lead for DxOMark Low-light ISO rankings. This result is obtained thanks to a slightly better SNR and a better Dynamic Range in comparison with the D3. The Canon EOS 1D Mark IV ranks 12th with a score of 1,320. So for high ISO, and after normalization, the difference between these two sensors is more than 1 f-Stop.”

Clearly, the pixel gain of the Mark IV comes at a price.