Sweeeet
I'm a freelancer, and I photograph all sorts of different stuff, and a 100-300 f/2.8 would be a perfect lens for my needs. If it comes with a built-in 2x TC (though the rumors doesn't mention it), it would be my dream lens.
I photograph a lot of wildlife and landscapes in the mountains, and this kind of lens would be a perfect compromise between weight, focal range, and a large enough aperture.
Also, when shooting outdoor sports, I'm usually limited as to how much gear I can lug with me (and a lot of times it is not really convenient to swap lenses in the middle of an action shoot anyway).
So I'm really excited for this one. I can't wait to see how it looks when it comes out and how it performs.