twistedphrame: yes I meant outdoor sports. I find IS to be very useful for longer lenses, I also find the 400 f/5.6 to be too slow even on my 5d II and the lack of weather sealing is another detractor. I find no faults with the lens' level of sharpness. I find there to be a large hole in Canon's telephoto lineup; I would love it if Canon would make a 400mm f/4 IS (non-DO) for ~2,500 with weather sealing.
deva207: I think an ultrawide would be fun, I've looked into canon's 14mm and am considering it in the future; it sounds like the mark II version has dramatically improved on full frame bodies. I think a fish-eye lens would be a fun one to rent, but I don't have many practical uses for it in my work. I have easy access to the 100mm macro (non-IS) for the rare-circumstances in which I need macro for my work but I find these situations to be rare.
sean: I definitely think I should invest in some.
peety3: I really like the 300 f/2.8; its just really expensive , I'd like to at some point use a 1Ds mark iv as my primary body and keep the 5d for when weight is a concern (travel etc). I love the 85 for studio work, the 200 is my dream lens but is somewhat limited in its use (not long enough for sports, too long for most portraits). The only wildlife I do is mostly when I am backpacking so I think the 500 is way too much for me.
Joel: portability is a major concern to me. I don't feel limited by my Speedlites at all, I'm always learning new things; perhaps in the distant future I'll have a full profoto setup. I think a converter is a great idea in the meantime; I'm a big fan of the 1.4 converter.
zVP: One could argue that the 16-35, 50, 70-200 to be the trinity of lenses; however, I find 24mm to be wide enough on a full frame body so I am willing to trade distortion for practicality. I can think of no flaws in any of the three. I hear both the 24-70 and the 70-200 are due for an update in January with the introduction of the new 1Ds. IS would be useful on the 24-70 during video operation, but unworthy of the upgrade cost otherwise. Improvements in flare control, vignetting, distortion, chromatic aberrations, sharpness are always welcome and I suppose I'd eventually upgrade if these were strong enough factors. Until then I find these lenses to be just about perfect for my uses.
Keith: I have rented the 100-400 and found that I used 400mm 90% of the time. For that reason I'd prefer a prime lens with a wider aperture.
conclusion:
So it sound like the following are items to save up for: 300 2.8 is, 85mm 1.2, 1.4 extender, CPL filter, second body, light-modifiers.