As of Nov9 2012, children travelling alone or accompanied by a third party of legal age ( grandparent, aunt/uncle, other ) most not only have a parental consent form but it "MUST be legalized by the Mexican Consulate or Embassy AND translated into Spanish".
I had heard of a local set of grandparents and their grandkids, being denied boarding in Vancouver heading to PV, just this afternoon. Since I am travelling with our niece ( in 9 days ) I thought I should double check what I need and it is true. I have the notarized forms from both parents but now have to scramble to translate and legalize them with the Consulate. The travel agents all seem to have just heard of this as well so if you are travelling with grandkids or nieces/nephews maybe double check.