Passports or other limited forms of ID will be required to enter the US
by air beginning January 23, 2007.

Persons arriving by land and sea (including returning US citizens)
*may* be *required* to cary a passport as early as January 2008.
Currently, most people presenting at Customs to enter the US are
carrying a passport voluntarily. The current law requires that the
person entering be able to prove "to the satisfaction of the Customs
Examiner" their country of origin and citizenship, and a valid passport
is a document that any Customs Examiner must accept as satisfactory.

Comments from the State Department:
(note: this information from the State Department Web Site reads that
the January 23, 2007 implementation is "proposed". The State Department
finalized the requirement on November 22, 2006)

New Requirements for Travelers


Under the Western Hemisphere Travel Initiative (WHTI)
The proposed implementation timeline has two phases:
· 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.
· 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.
Travel Documents for U.S. Citizens Under WHTI
Under the proposed implementation plan, the following documents will be
acceptable to fulfill document requirements:
· 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.