All the Canon lenses that have the designation EF-"S" (the "S" at the end) are meant for the 1.6 crop cameras, like the Rebel XT and XTi.
They will NOT work on a full-frame camera. That said, this lens is the stock lens that comes with the "package" deal for the new XTi.
Basically, it sucks. (OK, I'm spoiled with expensive lens)
They charge you like $300 extra and it is too slow a lens. Save the $300, get the body only and save a few hundred more to get a real medium zoom with a "L" designation (pro lens)... 85mm @ f5.6 is a waste of time.
I have a brand new 17-85 IS USM that I never use -- if you really want to buy one, pm me and I sell it cheap.
I never use it. Took less than 50 images with it and back in the box.
Below shot with Canon XTi with 70-200 f2.8 IS