In a previous post, I listed some links about passport requirements for air travel from Canada to the United States effective January 23, 2007. (The date for needing a passport to enter the U.S. by road is yet to be definitively established, as soon as January 2008, or as late as July 2009, depending on which politician or media outlet is discussing it.) UPDATE August 2008 -- See new date for passport post.
For Canadians, there is another option :: Frequent travellers can apply for the NEXUS Air Card for use at designated airports (most major airports across Canada will be added to the list).
Here are some useful sites for travel and passport information for Canadians :: Passport Canada and the new Western Hemisphere Travel Initiative (WHTI).