I'm not sure if the "Western Hemisphere Travel Initiative" that goes into effect on June 1 applies to others, but this is the reminder that was just sent to us this morning (I work for the US Postal Service, where passport applications are submitted):

"Beginning June 1, Americans will need to show a valid U.S. passport book, a U.S. passport card or other special travel document approved by the Department of Homeland Security when entering the U.S. by land or sea from Canada, Mexico, the Caribbean, and Bermuda at land border crossings or sea ports-of-entry."