Just went to the US Customs site and copied this, which I believe it is not in effect as of yet.
Documents You Will Need to Enter the United States
Important Note: The Western Hemisphere Travel Initiative will soon require all travelers to and from the Americas, the Caribbean, and Bermuda to have a passport or other accepted form of documentation to enter or reenter the United States. The program will be rolled out in phases. The proposed timeline is as follows:
[ul][*]As of 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.

*Lawful permanent residents (LPRs), refugees, and asylees will continue to be able to use their Alien Registration Card (Form I-551), issued by DHS, or other valid evidence of permanent residence status or refugee or asylee status to apply for entry to the United States. [/ul]

Travel Documents for U.S. Citizens Under WHTI
Under the proposed implementation plan, the following documents will be acceptable to fulfill document requirements:
[ul][*]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.[/ul]


Until this requirement is in effect as noted above, U.S. citizens are currently exempt from the requirement to show a passport unless they are returning to the U.S. from outside the Western Hemisphere (Europe, Africa, Asia, Australia). However, it is highly recommended that you be prepared to present documentation such as valid U.S. passport, U.S. birth certificate, or naturalization certificate to avoid delays.
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Swampthing you are correct However,
It does not make much sense not to have a passport in this case. If you drive up and have an emergency there will be fly out for youand besides they are an easy get.