Recently, United announced that they were no longer going to be allowing pets in the cabin to Mexico. Many of the US based carriers had already prohibited in-cabin pet travel on international flights. Delta, American, US Airways, and Air Canada will not transport a pet in the cabin to Mexico. It is unfortunate because these carriers dominate service in the south and southwest markets where the demand is the greatest. This has frustrated many pet owners who move back and forth several times a year to homes south of the border.
After doing some research, I discovered that Taca will fly pets in the cabin from Miami (MIA) to Cancun (CUN) or Monterrey (MTY). Other options from the south/southeast are from Dallas Fort Worth (DFW) to Mexico City (MEX) or Monterrey, Mexico (MTY) and Los Angeles (LAX) to Cancun (CUN) or Monterrey (MTY). Taca also flies out of Orlando, Chicago, Washington, New York, and San Francisco.
Sure hope this helps you pet travelers heading to south of the border!