The departments of Homeland Security and State of the US government have released changes to the Western Hemisphere Travel Initiative. The Initiative had required that all US citizens have passports in order to return to the US when visiting the Dominican Republic and other Caribbean countries as of 31 December 2005. The government announced it would extend the requirement to 31 December 2006. As of that date, the US government will require US citizens and foreign nations to present a passport when entering the United States. The requirement will apply to all air and sea travel to or from Canada, Mexico, Central and South America, the Caribbean and Bermuda. As of 31 December 2007, the requirement is extended to all land border crossings as well as air and sea travel.
See http://travel.state.gov/travel/cbpmc/cbpmc_2223.html