Well, being from Mexico (and living in Mexico) I think I can be of some help here. The conversion rate is about 10.5 pesos for 1 dollar (give or take, changes daily and it depends on where you trade your currency). Airport exchange rates are, as some have already said very high.
Now, the issue about bringing pesos or dollars depends greatly on waht part of Mexico exactly you are visiting. Yes, there are a lot of places that accept dollars (although they usually use their own currency exchange, which may be even worse than airports), but there are also places where dollars won't be accepted. Tourist areas prefer dollars, yes, but only because they are gaining more they would if you use pesos.
I do trust you know about certain political issues happening right now in Mexico if you're planning on visiting Mexico City or Oaxaca. (And since Eric's Granma doesn't like politics, I'll just leave the subject off of this and all my responses here).
Joe's advice is great, BTW.
Still, if you need more help feel free to drop me a PM or an e-mail (edheldur at yahoo dot com)