There have been many questions regarding the need for Passports while traveling from the United States to Mexico. Here is the most current official information we have:

Requirements for Travelers

• On January 8, 2007, ALL persons, including U.S. citizens, traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda are required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document.
  
• As of January 1, 2008, ALL persons, including U.S. citizens, traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), may be required to present a valid passport or other documents as determined by the Department of Homeland Security.

If you have any questions, please consult your local passport office or on the internet at:  http://travel.state.gov/travel/travel_1744.html