Effective October 1, 2007 U.S. citizens traveling by air to Canada, Mexico, the Caribbean and Bermuda must present a passport or other Western Hemisphere Travel Initiative (WHTI) compliant document to enter or depart from the United States. Prior to that, under a special exception, U.S. citizens could travel by air without a passport upon providing proof that a U.S. passport application had been made.

Press Release from Homeland Security