Peter Irons has now shown that the content of Dembski's latest book, Design of Life, shows that his explanation is a lie. That book includes a reference to the same video, with a link to its original location, marked as "last accessed" on January 25, 2007. Since he knew where the video came from in January 2007, he also already knew in September 2007. ERV points out the details of Dembski's deception.
UPDATE (December 31, 2007): There is an entertaining exchange of letters between Peter Irons, Bill Dembski, and Dembski's attorney John Gilmore posted at Pharyngula.