Traveling to Mexico?

is it true that you DO NOT need a passport to travel to Mexico (from the US)?

I know that used to be the case, but with all the crazy national security updates that the US has gone through lately, I wanted to double check as my girlfriend does not have a passport, and we are planning a vacation there...
NOTE: We will be flying in...
Thanks everyone. My girlfriend and I along with another couple are planning a trip, and they didn't think you needed one. I thought differently haha

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html

If you are flying in...you will need a passport. That started January 1st this year. It is different if you drive in. (For the time beeing.)

Regardless of Mexican laws, you will need a US Passport if you're travelling outside the country. New laws require that all american citizens must have a passport when travelling outside the country, and to return. Not having it can result in you being denied entry.

Starting in 2008, Mexico will require Americans to apply for a tourist visa to visit Mexico.

If you are FLYING to and from Mexico, you MUST have a passport. Requirement for a passport for LAND travel MAY go into effect as soon as next summer. Previous answerer gave you the website to read the current info at the state dep't site. See site below for all passport application info.