I took my Boston Terrier to PVR (Puerto Vallarta) in April 2009.
According to many websites, the Mexican Agriculture Dept requires rabies cert. No one was at the office upon arrival at PVR. No one questioned the dog in any way or asked to see any documentation. We just deplaned, walked through the terminal and out the front door where there is a patch of weeds below the pedestrian bridge over the highway. Literally it is less than 50m (160ft) from the front door of the airport. We actually crossed the highway and walked a few blocks (the dog was thankful) then took a taxi into town. I saved US$10 by not taking the taxi from the terminal.