Adorama has the Tokina AT-X 116 PRO DX-II 11-16mm f/2.8 for Canon for $499.90 (after $40 mail-in rebate). This is the absolute cheapest we’ve seen it ever! To be eligible for the $40 mail-in rebate it has to be purchased today, August 31st. This deal is also available at Adorama via eBay.
The regular price for this lens at authorized dealers is usually $639.