From the Washington Post, 6 April 2005:

Millions of Americans will be required to show passports when they reenter the United States from Canada, Mexico and the Caribbean by 2008 under new rules announced yesterday by the State and Homeland Security departments.

The new policy, designed to thwart terrorists from exploiting the relative ease of travel in North America, means that Americans who lack U.S. passports will have to obtain them to travel between the United States and neighboring nations. It also will require Mexicans and Canadians to present either passports or another official document to enter this country, with details to be determined.