You will have to have a valid US Passport to return to the United States from Mexico starting Jan 23 2007!
The Departments of State and Homeland Security has announced that the requirement for citizens of the United States, Canada, Mexico, and Bermuda to present a passport to enter the United States when arriving by air from any part of the Western Hemisphere will begin on January 23, 2007.
it is your responsibility to verify that your passport is current. This can be a lengthy process so please plan accordingly.
Read more about it at: http://www.state.gov/r/pa/prs/ps/2006/76752.htm