The 1DS Mark II was announced in Sept 2004, exactly 2 years after the original full frame sensor 1DS was announced in Sept 2002.
Quite possibly we will see an announcement of a new top of the line DSLR model from Canon on the 2 year anniversary of the 1DS Mark II, which will be in September of 2006.
Any big annoucements like this are made at the Photokina World Of Imaging exhibition in Cologne Germany. The Photokina show is also once every 2 years and this year it will commence on the 26th of September and run through the 1st of October:
http://www.photokina-cologne.com/If Canon has any big announcements planned for this year for certain they will happen in Cologne and not before.
www.dpreview.com usually provides a daily report online from Photokina during the show of all the new product announcements made at the show.