Mexico will never require a passport for simply crossing the border into their country. They may require a Visa if you plan on staying over a certain period or travelling a certain distance beyond the border.
This is not a U.S. policy as the U.S. does not inspect or challenge you when you LEAVE the country.
I'm done. Get ready to get passports for Canada, Mexico - 5starsnews.com
But thanks to the United States' ever-tightening security since the Sept. 11 attacks, Americans will be required to have passports to drive across the border by the end of 2007. Air and sea travelers will need passports even sooner.
That's passports to come back IN, not to LEAVE the U.S.