The Tokina AF 11-16mm f/2.8 AT-X Pro DX (review
) does have the potential drawbacks of being significantly more expensive and also not being compatible with Canon's full-frame bodies should a future upgrade be contemplated. The Canon EF 20mm f/2.8 USM (review
) has lower CA but doesn't seem to be as sharp.
After having a little rootle around it seems there's a real gap in the market for an EF/EF-S compatible and relatively fast quality prime lens of this focal length. Silly money will buy you a "ZE" Zeiss Distagon T* 2,8/21 later in the year and that doesn't even have AF.