I didn't need one when I went to Mexico.

Here is what I found on a website:

It is recomemended, but the following items are acceptable alternatives for U.S. and Canadian citizens when accompanied with a valid photo ID:


An original birth certificate

U.S. or Canadian naturalization papers

A notarized affidavit of citizenship

A valid voter registration card

http://www.tripspot.com/ask/mexicopassport.htm



(I don't remember showing any of those things when I went to Mexico or Canada though. I think I just showed my drivers license)