Starting Tuesday, U.S. citizens returning from Canada, Mexico or any other Western hemisphere country — via air travel — must present a passport. A drivers license or birth certificate is no longer valid.
United States officials, parrying complaints from Canada and Caribbean nations, said the measure was mandated by a 2004 law in which Congress adopted many proposals of the Sept. 11 commission. The purpose is to reduce the types of documents travelers can use to enter the United States, simplifying the job of inspectors looking for fake or invalid ones.
The new measure applies only to air travelers. Officials in the Department of Homeland Security said they expected to roll out the same restrictions for passengers arriving by land and sea by Jan. 1, 2008.
From the New York Times.