I had to make this decision also, and like many others have mentioned, I went for the 17-55mm f/2.8 IS USM. It has the best of everything.....L quality optics, wide constant aperture, IS to help when handheld, and also is wide enough to use as a walkabout lens. I found that the range between 17-24mm is really useful especially for landscapes and close quarters.
The main downfall of the 24-70mm L is that it doesn't have IS.....and it is a pretty big one considering the price. I'm not entirely convinced that the red 'L' status and build quality makes up for this.
The obvious downfall of the 24-105mm L is that it is f/4 and not f/2.8. This is even worse if you are considering it for low light situations.
The 17-55mm has both f/2.8 AND IS........and it has the extra focal range at the wide end........for low light situations and on a crop body, I'd say it's the best choice and fully recommend it. If you need extra reach, you can do what I've done and get the 70-200mm L for the extra reach at some point, because I doubt you will notice not having 55-70mm focal range anyway....less so than not having the 17-24mm range. The build quality is not as bad as people make it out to be........it's actually built the best out of all of the non-L lenses I've used, and the zoom is very smooth. Definitely doesn't feel cheap. Another bonus is that you can pick one up used for less than the 24-70mm f/2.8 L.
On a full frame body, obviously it would be different, but on a crop sensor the 17-55mm is hard to beat, and many people write it off because it doesn't have the red 'L'.