I just read on another board that the Mexican Govt has passed a law that vendors can no longer accept US Dollars for purchases of more than $100. They will accept American bank backed credit cards (VISA, Master Card, etc.) for purchases over that maount. It appears the govt is trying to stem the flow of US Dollars from the drug cartels in hopes of bringing the drug problem under control.
I've been in touch with friends in Mexico about this, and it is still business as usual with US tourists. US dollars are still welcome in tourist areas. If you want to get anything really expensive, you're probably best off using a credit card. No need to carry tons of cash when ATMS are readily available and everyone except very small vendors will take credit cards.
Just remember that whether you are purchasing with pesos or with dollars, you will get pesos in exchange.