Q: When will the travel initiative be implemented?
A: The initiative will be implemented in two phases. The timeline is as follows:
* Phase 1: Beginning January 23, 2007, U.S. citizens and citizens of Canada, Mexico and Bermuda traveling by air between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda will be required to present a valid passport to enter (or re-enter) the U.S.
* Phase 2: As early as January 1, 2008, U.S. citizens traveling between the U.S. and Canada, Mexico, the Caribbean, and Bermuda by land or sea (including ferries), could be required to present a valid U.S. passport or other documents as determined by the Department of Homeland Security in a forthcoming separate rule. 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 acceptable documents for land/sea entries.
Quoted from:
FAQs on New Requirements - CBP.gov