B&H Photo Video has the optically-stabilized Sigma 17-50mm F2.8 EX DC OS HSM for Canon for $399.00. This is the lowest price we have seen from an authorized dealer. B&H is also offering a 4% (or $16) reward. This deal ends on December 5 (tomorrow) at midnight. As usual, the B&H website is open but checkout is closed from Friday evening to Saturday evening.
Amazon also has the Sigma 17-50mm F2.8 EX DC OS HSM for Canon for $399.00. No deal expiration date is mentioned on the page.