Come January 23, 2007 and all persons, U.S. citizens included, who will travel by air between the United States and Mexico will have to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document, or an Alien Registration Card, Form I-551, if required. Come January 1, 2008 and all persons, U.S. citizens included, who will travel between the U.S. and Mexico by land or sea (including ferries), may have to present a valid passport or other documents (NEXUS, SENTRI, FAST, Passport Card, etc.) as determined by the Department of Homeland Security (DHS). The United States Department of State and the DHS are sparing no efforts to meet all requirements as soon as possible though a postponement of deadline has been permitted by recent legislative changes. Travelers have been assured that advance notice will be provided so that they are not inconvenienced and can obtain passports or passport cards for land/sea entries.
And after December 31, 2007, all US citizens must produce a passport or passport card if they want to take a road trip into Mexico. Mind you, even if you can enter Mexico without a passport, you won't be allowed to return to the United States without one.
If you wondered what was wrong with the old system, you should know that all these new rules have been designed to prevent potential terrorists from entering the United States through other North American countries. These rules will affect every US citizen entering the United States from countries within the Western Hemisphere, such as the Caribbean, Bermuda, Central and South America, Mexico and Canada. All this is part of the Western Hemisphere Travel Initiative (WHTI) which was formed by the Department of State and the DHS together saying that by the end of 2007, US citizens will have to show a passport, passport card or other accepted document for all air, sea and land border crossings.

Remember that your driver's license won't be enough to prove who you are and get you back to the United States. The US authorities are, however, considering accepting other "appropriate" documents like the new passport card, the SENTRI program card or the FAST program card. And if you happen to be a Mexican citizen traveling into the US from a contiguous country, you may be allowed to use the Border Crossing Card (BCC, also known as the "laser visa").

Launched a few years back, the SENTRI (Secure Electronic Network for Travelers Rapid Inspection) allows prescreened applicants and vehicles to cross the border heading north into the US more quickly and efficiently, thanks to commuter lanes reserved just for them. The FAST (Free And Secure Trade)card enables drivers, fleets, importers and border manufacturers to get a quicker, "low-risk" US/Mexico border clearance (even if there is high security alert). This program is particularly suitable for truckers who have to make frequent border crossings. Most Mexican visitors to the US obtain a biometric Mexico Border Crossing Card (BCC), which is a laminated, machine-readable credit card-type of document with many security features and a10-year validity. This card works as both a BCC and a B1/B2 visitor's visa.