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Chris, I have owned the 24-70, currently own the 17-55 and the 10-22, and have taken thousands of photos with each of them. These are my experiences, but keep in mind that everyone has different shooting styles and will have different preferences on focal lengths. I have only ever owned a crop camera, 350D, 30D, and 40D. This advice would be different if you planned on getting a full frame in the near future.
The 24-70 is a wonderful lens, but I found it just was not wide enough on a crop camera. I had the 10-22 at the same time and was constantly changing lenses, particularly when travelling.
Ended up selling the 24-70 and getting the 17-55. Couldn't have been more happy. It is a wonderful lens too, and the focal length range is much, much better suited to crop cameras as a standard walk-around zoom lens. It will become the lens you have attached to your camera all the time, and you only take it off for special shots like ultrawides with 10-22 for example, or your bird shots 400mm.
Do not worry about the gap in focal length range between 55mm and 70mm. You will not even notice it. I didn't even have a 70-200 and made do with 100-400, so the gap was 55-100.
Do not worry about the comments on build quality. It does not feel plastic at all. It's actually quite a heavy and big lens - lot of glass. There is a little bit of dust in mine, but not much and it does not affect shots in any way. It's not like dust on your sensor where you get dust spots in your shots.
The 10-22 is also a wonderful lens, but is a very different beast to the 17-55. I would not recommend it as a standard walk-around lens. Too wide. Use it more for landscapes, waterscapes, star trails, or special effects like shooting up close and getting funky almost fish-eye type distortions.
I see a recommendation for the 17-40 too. It is also a great lens. I haven't owned one, but my brother has it and I have used it quite a bit. It's only f/4, compared to the 17-55's f/2.8, and it doesn't have image stabilisation like the 17-55. I would say the 17-40 (and the 16-35L) would be better on a full frame camera for taking landscapes and would be the "equivalent" of 10-22 on a crop camera.
So, in summary, I'd get the 17-55. It's definitely a cut or two above the 17-85 you currently have.
Next lens I'd recommend after you get that is the Canon 10-22 or the Sigma 10-20 for those ultra-wides. These will complement, rather than replace, the 17-55.
Just my, very long, 2 cents worth.
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