Here is the exact wording from the Embassy of Mexico ...
"According to the new regulations, from the 1st of May 2010 it is not necessary to apply for a tourist, business or transit Mexican visa if you hold a VALID VISA for the United States of America -regardless the nationality- . Immigration authorities in Mexico will ask for your US visa and a valid passport when you arrive. "
"Permanent residents in Canada, Japan, The United Kingdom (Indefinite Leave to Remain in the UK), The United States of America or the Schengen countries wishing to travel to Mexico do not require a visa to enter the country as a tourists and business visitors for a 180-day stay and as a visitors in transit for a 30-day stay."
This is an extract from an email I got from the Consulate of Mexico in Washington DC: "If you have a valid passport (any nationality) and a valid US visa (any kind of visa) or green card, you do not need a tourist visa to visit Mexico"
Adding my first hand experience below
The wife and I traveled to Mexico in August 2012. We have US L1 and L2 visas, and we also have older B1/B2 visas. The Immigration Officers at Cancun Airport just looked at our US B1/B2 visas, stamped our passports and let us in. No questions, no hassles, no need for a Mexico visa.