2008 U.S. Passport Information: New federal regulations require all U.S. citizens to have passports for air travel to and from Mexico. Cruise passengers and travelers crossing by land will also need passports beginning January 31, 2008.
2009 U.S. Passports and U.S. Passport Card Requirements: The new regulations have prompted a passport rush. The State Department is warning Americans that they won't be allowed to enter the country from Mexico without a passport after June 1, 2009
The U.S. government has also introduced passport cards designed to speed border crossings by U.S. citizens traveling to Canada, Mexico and the Caribbean. The government approved the wallet-size cards as a cheap and convenient alternative to passport books. They may be used only by citizens returning to the United States through land or seaports from Mexico, Canada, Bermuda and the Caribbean. Air travelers must carry full passports.
The cards emit a radio signal picked up by Customs and Border Protection officers at checkpoints. The signal provides inspectors with a file number linked to passport data on each cardholder. The cards are handy for frequent border crossers, and cost just $45 for a first-time adult applicant, rather than the $100 fee for a passport book.
Traveling to Mexico by Car: Mexico strictly regulates visitors entering the country by car. Before driving across the border, you must have the vehicle's state registration certificate certifying legal ownership or the leasing contract. If the vehicle is leased or rented, it must be in the name of the driver. If the vehicle belongs to a company, proper documentation is necessary to show you work for the company.
U.S. automobile insurance is not valid in Mexico, so you'll have to purchase a separate policy covering the length of your stay. If you're in an accident, you will be taken into police custody until it can be determined who is liable.