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The Mexican consulate in San Francisco says that you do not need a visa as long as you have valid and unexpired USA visa (or permanent residence).
Other nationalities, Non-US Permanent Residents. Starting May 1st, 2010, all those foreign nationals, regardless of their nationality, visiting Mexico for tourist, business or transit matters won't need a Mexican visa as long as they have with them a valid and not expired USA visa in the passport. In this context, if you have a valid US visa in the passport you do not need to obtain a Mexican visa.
The website is here.
Similar, but more clearly stated information is available on the Consulate of Mexico in India website which says:
:: NOTICE FOR HOLDERS OF USA VISA ::
Holders of any valid visa for the United States of America (any nationality) DO NOT require visa to enter Mexico on tourist, business and transit purposes only.