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• Beginning January 23, 2007, ALL persons, including U.S. 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 U.S. Coast Guard Merchant Mariner Document, or an Alien Registration Card, Form I-551, if applicable.
  
• As early as January 1, 2008, ALL persons, including U.S. citizens, traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), may be required to present a valid passport or other documents as determined by the Department of Homeland Security. While recent legislative changes permit a later deadline, the Departments of State and Homeland Security are working to meet all requirements as soon as possible. Ample advance notice will be provided to enable the public to obtain passports or passport cards for land/sea entries.

• U.S. Passport: U.S. citizens may present a valid U.S. passport when traveling via air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda, and may also use a U.S. passport when traveling via sea and land borders (including ferry crossings).
  
• The Passport Card (also referred to as the PASS Card): This limited-use passport in card format is currently under development and will be available for use for travel only via land or sea (including ferries) between the U.S. and Canada, Mexico, the Caribbean, and Bermuda. Similar in size to a credit card, it will fit easily into a wallet.
  
• DOS and DHS also anticipate that the following documents will continue to be acceptable for their current travel uses under WHTI: SENTRI, NEXUS, FAST, and the U.S. Coast Guard Merchant Mariner Document. As proposed, members of the U.S. Armed Forces on active duty traveling on orders will continue to be exempt from the passport requirement.

Background

The Intelligence Reform and Terrorism Prevention Act of 2004 requires the Department of Homeland Security and Department of State to develop and implement a plan to require all travelers, U.S. citizens and foreign nationals alike, to present a passport or other document, or a combination of documents, that denote identity and citizenship when entering the United States. Congress amended portions of the Act in 2006. The Western Hemisphere Travel Initiative is the Administration's proposed plan to implement this mandate.

The goal of the Initiative is to strengthen border security and facilitate entry into the United States for U.S. citizens and legitimate foreign visitors by providing standardized, secure and reliable documentation which will allow the Department of Homeland Security to quickly, reliably and accurately identify a traveler.

Source: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html