I can't say I've personally attended any film festivals in Mexico but your question was so intriguing that I couldn't resist doing a bit of research on this topic.

The following film festivals based out of Mexico sound quite promising and their profiles seem to be on the rise as Mexican directors and actors have become successful in stepping into the international spotlight over the past 8 years:

-Short Shorts Film Festival Mexico http://www.shortshortsmexico.com.mx/

-Mexico International Film Festival http://www.mexicofilmfestival.com/

-Morelia International Film Festival

-Hola Mexico Film Festival (BASED OUT OF NEW YORK) http://www.holamexicoff.com/

-Mexico City International Documentary Film Festival (DOCSDF) http://www.docsdf.com/English/index.html

-FICCO (Spanish acronym for City of Mexico's International Festival for Contemporary Cinema) http://www.ficco.com.mx/

-Guadalajara Mexican Film Festival (Guadalajara is a focal point for seekers of a Mexican culture fix) http://www.guadalajaracinemafest.com/english/index.php

-Cine Pobre / International No Budget Low Budget Film Festival (in La Paz, Mexico) http://www.cinepobre.com/

-Cancun International Film Festival http://www.cancuninternationalfilmfestival.com/en/

-Baja California International Film Festival (in Ensenada, Mexico) http://www.lamiafoundation.org/bajacaliforniafilmfest.html

This list might overlap with some of the selections above:

MEXICO

-Cd. Juarez, Chihuahua (September/November): Festival de Video de la Frontera Norte, Combo
http://www.combojuarez.com/

-Guadalajara (March): Festival Internacional de Cine en Guadalajara
http://www.guadalajaracinemafest08.com/es/

-Guanajuato (July): Expresion en Corto - International Film Festival
http://www.expresionencorto.com/web/

-Matamoros, Tamaulipas (September): Cine En Corto (short films) http://www.cineencorto.com/

-Mexico (Travels to 16 different cities in Mexico) (February-April): Ambulante Gira de Documentales
http://www.ambulante.com.mx/2008/es/home.php

-Mexico City (September/October): International Documentary Film Festival of Mexico City: DOCSDF
http://www.docsdf.com/

-Mexico City (September): Short Shorts Film Festival Mexico
http://www.shortshortsmexico.com.mx/

-Mexico City (February): International Contemporary Film Festival
http://www.ficco.com.mx/

-Monterrey (August): Monterrey International Film Festival
http://www.monterreyfilmfestival.com/

-Morelia (October): Morelia International Film Festival
http://www.moreliafilmfest.com/2008/ES/

-Rosarito (April): Mexico International Film Festival
http://www.mexicofilmfestival.com/

I am guessing that this is by no means an exhaustive list for the country but it might give you a head start to discover smaller regional events that may not have the publicity budget to really spread the word.