Just got back from Socorro on the Solmar V (great trip as usual) and got hassled from Mexican customs again. This time US Airways only had the old customs forms, which allows only one camera and one video. So the customs agents told us they hold us to whatever form is used, regardless of what the rules really are. After arguing with the customs agent about the ever changing rules, he let me go.
So in the event you get stuck with an old form from your airline, be sure to pick up the new form at the airport you fly into. I would also pick up a couple of extra forms and keep them for future use in the event they find another box of old forms laying around that they decide to use instead of toss.
I believe that in Mexico what goes today may not go tomorrow but will go the next day. It's virtually a crap shoot, you are at the whim of the people that are messing with your gear. One thing is helpful, have your camera clearly marked with your name on it using a tape gun, let's them know you aren't trying to sell the stuff.