Should this CR2 rumour become reality, I can see an improved Canon 100-400mm L being very popular with a variety of shooters. I believe particularly those who are looking for a flexible, high quality telezoom to capture a variety of wildlife (eg from birds to wildlife in games parks, etc).
I have the Canon 70-300mm L, and love that lens. One of the things I really like about the lens is it's zoom range : portability aspect. While I would love a 100-400mm L, I was 'put off' by the bulk and impracticality for transport, having handled the current 100-400mm L zoom and some of the similar Sigma lenses (eg 120-400mm, 150-500mm). When I go travelling (locally or further afield), this is usually what I take in my LowePro shoulder bag: Canon 7D body, 15-85mm and 70-300mm L lenses. It's a great travel kit.
The 'soonish' in the rumour is an interesting aspect.
I could imagine if it was released soon, a lot of the new 100-400mm lenses would be used for the major 2012 sports (eg Olympics). However I know there will be people who don't necessarily need the 400mm (or who, like me - are happy 90% of the time cropping at 300mm to obtain pleasing results).
I'm aware there are people who are considering the 70-300mm L which has some of these features / benefits compared to the current 100-400mm L ie improved IS, zoom mechanism, alleged slightly improved AF & IQ. If a new 100-400mm L comes out, it may be the
lens they are after if it 'borrows' or migrates those features from the 70-300mm L.
Should the alleged price $2800 USD in the CR2 rumour is accurate for the 100-400mm L, purchasers will be 'paying for this' in the upgrade over the existing 100-400mm L.
I paid AUD$1600 for my lens soon after it was released, and I'm very happy with it. My copy has stellar IQ corner to corner: sharp, contrast, very low CA, etc.
My thoughts are that I wouldn't be surprised if the folks at Canons put out the 70-300mm L, and kept a very close eye on the sales, to determine the 'last minute tweaks & production' to a new 100-400mm L design / model they had in the pipeline already.
If I would want to get really REALLY super serious into birding (or wildlife) photography - even as a hobby - and felt it wouldn't be too selfish, the 200-400mm f/4 with built in 1.4x teleconverter, (thus also being a 280-560mm f/5.6) presents a much more desireable lens to me because of the additional reach, still at aperture 5.6. It would match my 7D very well. However at this stage I have no plans to buy such an expensive and large lens.
These are interesting times. Thanks Canon Rumours for what you share, and for this forum. More choice can't be a bad thing. Horses for courses, and I do hope any new 100-400mm L would make for many happy photographers!