http://travel.state.gov/travel/cbpmc/cbpmc_2223.html
Beginning January 23, 2007, ALL persons, including U.S. citizens, traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda will be required to present a valid passport.
As early as January 1, 2008, ALL persons, including U.S. citizens, traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), may be required to present a valid passport or other documents as determined by the Department of Homeland Security