I've been shooting with CANON cameras for about 5 years. Everything from 10D to 1DIII. D3 made me want to try Nikon.
I got D3 and bunch of Nikon glass and took it to the wedding, along with Canon 1DIII. I ended up using D3 for about 75% of the images.
Really there is no comparison to 1DIII. I didn't need any post processing on D3 files, maybe slight WB or contrast adjustment here and there, that's about it. Auto WB is amazing, so is the noise(or lack of) at ISO3200. I loved this camera and can't find any flaws in it so far. I'm sure Canon will come up with something to stay in the game, but there is nothing out there right now that comes close.
If money is not an issue and you're deciding Canon/Nikon... D3 is the way to go. Next best thing (for wedding photographer, which I am ) is 5D, or two of them for the price of D3.