You're right Jason - I made a mistake here - the lens I was thinking of was the 28-300mm f/3.5-5.6L IS USM, which Humayun and I came away with the impression that it was heaver-still than the 100-400 by almost 300g. Given that at the time I decided against it - and I also thought that it may be compromised at either extreme of zoom in terms of overall image quality.
The 70-300's still on the cards though for me. I know a fellow who has one, a professional outdoor portrait/model photographer, and he likes it very much. Sounds like the ticket to me.
