The US Department of State has announced that as from 23 January 2007, all persons, including US 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, Air Nexus card or US Coast Guard Merchant Mariner Document. It had been announced that the measure would come into effect on 8 January 2007, but it was postponed until the 23rd of January. The US has authorized an extension until 1 January 2008 for people traveling by sea between the US, Canada, Mexico, Central and South America, the Caribbean and Bermuda.
http://travel.state.gov/travel/cbpmc/cbpmc_2223.html