As of Tuesday, all air travelers coming into the United States - including returning U.S. citizens - will be required to have a passport.

Until now, travel to the U.S. from Canada, Mexico and the Caribbean didn't require a passport.

According to the Department of Homeland Security, the passport requirement will apply to truckers and four-wheelers entering the U.S. from Canada or Mexico as early as January 2008, although many are speculating that the date may end up being pushed back.