Western Hemisphere Travel Inititiative

Effective January 8, 2006, the Western Hemisphere Travel Initiative (WHTI) will require all travelers to and from Canada, Mexico, Central and South America, the Caribbean and Bermuda to present a passport or other accepted document that establishes the bearer’s identity and nationality to enter or re-enter the United States. The goal is to strengthen border security and facilitate entry into the United States for U.S. citizens and legitimate international travelers.

Check out the U.S. Department of State's Web site for more details.