According to Consulado de Mexico en Miami:
With the purpose of simplifying and promoting traveling to Mexico, starting May 1st, 2010:

All foreign visitors who travel to Mexico as tourists, on business, or in transit to another country will not require a Mexican visa, regardless of their country of citizenship, as long as they have a valid passport and a valid US visa (which must be presented before Mexican Immigration authorities at the Port of Entry).

http://portal.sre.gob.mx/miami/index...=page&SubMenu=