So you want an ultra-wide lens AND you want a fast f/2.8 aperture? Well, the Tokina 11-16mm is the one for you. It doesn't have a very wide zoom range, but it has a lot of positives: it's fast, very well-built, and it's very very sharp: probably the sharpest ultra-wide lens available for cameras with APS-C sized sensors. And you've probably got a zoom starting at 17mm or 18mm anyway; this is just for the really wide shots.
COMPATIBILITY NOTE: This lens only fits the following EF-S mount cameras: 7D, 20D ... 60D, 300D ... 650D, 1000D ... 1100D. It does not fit on the 6D, 5D series or 1D series cameras.