Effective 06/01/09 all US citizens must have a valid passport to re-enter the US.
A 2004 law is phasing in new rules for travel between Canada and the United States. Since January 2007 U.S. citizens traveling by air to and from Canada have needed a valid passport to enter or re-enter the United States. Beginning January 31, 2008, U.S. and Canadian citizens aged 19 and older traveling into the U.S. from Canada by land or sea (including ferries) have had to present documents denoting citizenship and identity. This change primarily affects American and Canadian citizens who have previously been permitted entry into the U.S. by oral declaration alone, and marks the transition toward standard and consistent documents for all travelers entering the U.S.
Acceptable documentation includes a valid passport or government-issued photo identification such as a driver’s license and proof of citizenship such as a birth certificate. Children aged 18 and under need only to present a birth certificate. A list of acceptable documents is found at http://canada.usembassy.gov. Travelers who do not present acceptable documents may be delayed as U.S. Customs and Border Protection officers at the port of entry attempt to verify identity and citizenship. Beginning in June 2009, all travelers, including U.S. citizens, will have to present a passport or other secure document that denotes identity and citizenship when entering the U.S. from Canada.
Full disclosure information can also be found on the U.S. State Department website at http://travel.state.gov/travel/cis_pa_tw/cis/cis_1082.html.