From the Dept. of State website:
ENTRY REQUIREMENTS: The Government of Mexico requires that all U.S. citizens present proof of citizenship and photo identification for entry into Mexico. However, some U.S. citizens have encountered difficulty in boarding flights in Mexico without a passport. The U.S. Embassy recommends traveling with a valid U.S. passport to avoid delays or misunderstandings. A lost or stolen passport is easier to replace when outside of the United States than other evidence of citizenship. However, U.S. citizenship documents such as a certified copy (not a simple photocopy or facsimile) of a U.S. birth certificate, a Naturalization Certificate, a Consular Report of Birth Abroad, or a Certificate of Citizenship are acceptable. U.S. citizens boarding flights to Mexico should be prepared to present one of these documents as proof of U.S. citizenship, along with photo identification, such as a state or military issued ID. Driver's licenses and permits, voter registration cards, affidavits and similar documents are not sufficient to prove citizenship for readmission into the United States.
The Intelligence Reform and Terrorism Prevention Act of 2004 requires that by January 1, 2008, travelers to and from the Caribbean, Bermuda, Panama, Mexico and Canada have a passport or other secure, accepted document to enter or re-enter the United States. As of December 31, 2006, this requirement will apply to all air and sea travel to or from Mexico.