There's another option for APS-C format shooters (initially offered in Canon, Nikon and Sigma mounts), this time with a constant and fast, f/1.8 aperture:
PopPhoto: Sigma 18-35mm f/1.8 DC HSM
I do have mixed feelings about it, mainly its size/weight and potentially cost but kudos to Sigma for breaking a new ground. Now, go build a better 20mm f/1.8 "DN".