Bah, I'd read about that elsewhere and forgotten. Guess the rear focus elements would hit the extender's optics.
Agreed totally on the balance of the new lens versus the multi-lens solution. What's more, it's simply a newer lens and the other factors (new IS system, weather and dust sealing, better ergonomics) make me feel it's a reasonable investment compared to older designs.
Another bad thing with this lens, as revealed by TDP, is that it's already at f/5.0 way before 200mm, at 155mm in fact.
Thanks for the link and information - I wasn't expecting Bryan C. to update from the placeholder page so quickly! Guess my faith he's an insider was well placed. Anyway, I was afraid that the maximum aperture would become rather low quite quickly, though that is rather more extreme than I expected. I'm not sure it will matter to me though - for 300mm, for instance, the sole affordable choice (for me) besides other zooms is the aging f/5.6 L single-focal length design, which I was considering previous to the 70-200mm (as mentioned before). If I need a faster 200mm lens, there are some good single focal length options available such as the 200mm telephoto, or the 180mm macro. I do think that continuing improvements on ISO and the tradeoffs in the AF performance of lower-tier DSLRs means that this will probably be a better match for those cheaper systems, at the expense of a more artistic look from faster lenses and more noise in low-light pictures.