just to clarify - you can only use the EF-S lenses on the EOS DSLR bodies that have the smaller mirrors (in terms of age I think it's the 300D / 20D onwards, including the 350D/XT, 400D/XTi, 30D and 40D)
If you try and use it on a 10D, for example, or on a 5D or film body then the mirror will hit the rear lens element - potential damage! On the 5D you wouldn't be getting a full image anyway - the image circle covered by the lens is smaller than a full frame sensor. I think it may be just too small for the 1D series as well.
The mirror issue doesn't apply to Sigma's DC lenses (don't confuse with DG - DG fits and works on full frame) but you will still have the smaller image circle problem on, say, a 5D. For that body you're really looking at the 24-105L (or, on a smaller budget perhaps the 28-135 IS) to get a similar field of view - not cheap (but great images!).
The 17-40L would give you the same wide end on the XTi as the 17-85 would, but you don't get the "longer end". Plus if you move the lens to a 5D it becomes VERY wide angle at the 17mm end, and you have even less at the "long end"
A 28-135 gives the same field of view on a 5D that the 17-85 gives on an XTi, so you'd have to change lenses anyway, in reality, unless you want to shoot differently!
Hope that helps a bit?.....
PS one lens that gives a very wide angle on the XTi and then EXTREME wide angle on the 5D is the Sigma 12-24 DG. However that is NOT an every day all-purpose lens