I have to disagree with neuro. I don't think the 70-300L qualifies as a replacement for the 100-400.
The main reason most people buy the 100-400 is precisely to be able to get to 400mm ! The strenght of this lens is the long range.
Just to clarify, I don't think it's a replacement. My point was that I think that Canon thinks it's a replacement.
I think there is also a fair amount of marketing spin on that as well. Just like the 1Dx being a replacement for both the 1D and 1Ds lines, I don't buy it. I would say - Canon is saying its a replacement - for now, but that is not their long-term position. We will see a high MP 1Dx at some point. I'm not so sure with the 100-400 vs. the 70-300L. Canon's argument is a little stronger on that one.
Having said that, the 70-300L vs 100-400L is a difficult choice. The 70-300 has all the improvements in IS and AF, but you lose the long end. I want to get one or the other at some point for sports (cross country, track). Anyone have experience with both and have suggestions?