Being realistic, the Nikon would most likely out perform the Canon.
Realistic? What ever do I mean?!
The Canon 70-200 f/4 IS was released in 2006. No doubt a great light and more affordable option compared to the f/2.8, but it is a little old (I wouldn't mind Canon releasing a mark II of this, to be honest. the f/4 IS is so outdated now, although a great option). Even though it does perform magnificently with it's optics, AF, etc., the Nikon would most like be using more recent technology. I guess we'll see with comparisons, but Nikon should take the cake because of how new it is.
I'll just wait and see what it's like though (not insinuating that I'm going to switch to Nikon. NEVER WOULD I!).