I heard that as of January 1ST of 2007 any American citizen traveling out of the USA would have to carry an American Passport. I know it takes somewhere between 4 to 6 weeks to receive the Passport once you apply for it.
Can a Naturalized Citizen travel to a Mexican Border city While Passport is in process?
Effective January 23, not 1st, US Citizens are required to have a passport to REENTER the US if flying. However, if you are crossing into Mexico from a land border by car or on foot, you will not be required to have a passport until 2008. The sooner you apply for a passport, the better.