I agree with the Tamron option. Though looking at the ISO charts, the VC version of the lens doesn't seem anywheres near the quality of the non-VC. My dad has the non-VC 17-50 f/2.8, and it's super sharp. Unbeatable for the price. At around $450 it compares extremely well with the Canon EF-S 17-55 f/2.8 IS, which is over twice the price. Highly recommended for a fast midrange zoom.
Derrick