While we encourage travel and touring into Baja California, Mexico.  Rules and regulations are changing constantly and we inform all our clients to possess a passport for re-entry into USA.   From the United States State Department Website on the Western Hemisphere Travel Initative:

AIR TRAVEL

• All persons traveling by air outside of the United States are required to present a passport or other valid travel document to enter or re-enter the United States.

LAND AND SEA TRAVEL

The following summarizes information available on the Department of Homeland Security's website.

• CURRENTLY: 
  All U.S. citizens must show proof of identity and proof of U.S. citizenship when entering the United States from Canada, Mexico, Bermuda and the countries of the Caribbean by land or sea.
  
  Acceptable documents include: U.S. Passport Book, U.S. Passport Card, or other document approved by the Department of Homeland Security.
  
  U.S. citizens who do not have a single document verifying identity and citizenship must present both an identification and citizenship document; for example, a driver's license and a copy of a birth certificate or naturalization certificate.
  
  
• LATER:
  On June 1, 2009, the U.S. government will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have a passport, passport card, or other travel document approved by the Department of Homeland Security.
  
  
  • CHILDREN: U.S. citizen children under the age of 16 will be able to present the original or copy of their birth certificate, or other proof of U.S. citizenship such as a naturalization certificate or citizenship card.
    
    Groups of U.S. citizen children ages 16 through 18, when traveling with a school or religious group, social organization, or sports team, will be able to enter under adult supervision with originals or copies of their birth certificates or other proof of citizenship. See the Department of Homeland Security's GetYouHome.gov for more information on the changing travel requirements.
    

U.S.PASSPORT AND WHTI COMPLIANT DOCUMENTS: 

• U.S.Passport: U.S. citizens may present a valid U.S. passport when traveling via air, land or sea between the U.S. and the aforementioned Western Hemisphere countries.
• The Passport Card: Passport card applications are currently being accepted in anticipation of land border travel document requirements. Based on current projections, we expect the passport card to be in full production beginning in July 2008. We will provide additional updates as available. Once in production, the passport card it will only be valid for land and sea travel between the U.S. and Canada, Mexico, the Caribbean region, and Bermuda.
• WHTI-Compliant Travel Documents for U.S. citizen travel via land or sea, as of January 31, 2008:
  • Trusted Traveler Cards (NEXUS, SENTRI, or FAST)
  • State Issued Enhanced Driver's License (when available)
  • Enhanced Tribal Cards (when available)
  • U.S. Military Identification with Military Travel Orders
  • U.S. Merchant Mariner Document when traveling in conjunction with official maritime business
  • Native American Tribal Photo Identification Card
  • Form I-872 American Indian Card

For further information see U.S. Customs and Border Protection's website Ready, Set...Go!

Please visit this link for more information: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html