Canon 1D Mark IV...photo taken with the Nikon D3s at ISO 12800. Click here to download the full resolution photo.
When the Canon 1D Mark IV was announced, I was a bit disappointed: it is essentially a 1D3 with more megapixels (16 mp vs 10 mp), new AF system and video. That said, the Mark IV promised out of this world high ISO capabilities (it reaches ISO 102,000!!!). The previous 1D3 maxed out at 6400, so if the 1D4 had really a 4 stop improvement, it would have been a real winner.
In January 2010, my friend Sandro loaned me his brand new 1D4, and I compared it with my cameras (Canon 1DsIII and Canon 7D) and with its direct competitor, the Nikon D3s.
 
Canon vs Canon: 1D4, 1Ds3 and 7D
In this test, I have compared the three cameras at every ISO from 100 to 12800. These images are 100% crop from RAW files, converted with minimum contrast, no noise reduction nor sharpening. Exposure was the same for every camera; the crops have not been resized, so the 1Ds3 crop look more magnified, due to the higher resolution of the 1Ds3. You can see two sets of crops: one from a shadow area, and another one from a well lit area.