17-40: You will definitely lose IS and the longer end. I dunno, I like a bit more telephoto on my walkaround zoom, but if you can live with the focal length, the quality is definitely there. The 17-40 on APS-C has definitely got better image quality wide open than the 17-85, especially wide open at 17-24mm.
17-55 f/2.8: awesome lens with awesome specs. Color, contrast, performance all L grade. I am a little put off by the dust problem though.
16-35 f/2.8: L and everything. Even shorter focal length but I have seen wedding professionals use this on a 20D well before the 17-55 f/2.8 came out. I suppose they have their reasons..
24-105/24-70 L zooms: Awesome image quality, awesome performance, everything you want from an L. *NOWHERE* near wide enough on APS-C at least for me. Crop factor turns 'em into 40mm zooms at their widest end. Nope, doesn't work for me. 24-105 lives on 5D.
I'm in a similar situation, JohnnyG. I pretty much agree with Lightstream above.
On the whole I'm pretty pleased with the results I get from the 17-85. The range it gives is fantastic for genreal walk-around, and I've employed the IS to get some shots at some pretty low shutter speeds without a tripod. However, I'm looking to replace it as I'm not too keen on the edge sharpness (zoomed wide), CA under certain circumstances & to a lesser degree the light fall-off (esp. wide open). This move had been on the back burner, but I've just missed a couple of trips for medical reasons and the insurance plus resale of 17-85 should (hopefully) cover a new lens.
I had been thinking of the 17-55, but I must admit the dust problem puts me off. I also agree this is a little irrational; my ~18 month old 17-85 has a collection of lint/dust on the first (last?!) optical element inside, but I'm sure it has no perceptible effect on IQ (I just know it's there - and it bugs me 
). I would have to get the hood too (more cost) and would like a pouch (even more cost). The f/2.8 & IS, plus the IQ are a big draw, but the build (I guess similar to my 17-85) and dust are a put-off.
I have used my father's 17-40 and do like the feel (build) and IQ, although I (think I) would miss the top 45mm from my 17-85, and think I would miss the IS in certain situations (haven't used the L enough to know).
I must say I'm tempted with the 16-35, but I don't know if I could live with the 35-70 gap that would leave (although I do have a 50). I would however be happy with the build & IQ . This comes with the hood & pouch too, of course. Again, might miss the IS, but not as much as f/2.8 & shorter range
At the moment I would say I'm about 20% 17-40L, 42% 17-55 Ef-s and 38% 16-35L. 
