Here is what has been posted on the government website...Sounds like if you avoid these areas, all will be fine:
"While millions of U.S. citizens safely visit Mexico each year (including thousands who cross the land border every day for study, tourism or business), violence in the country has increased recently. It is imperative that travelers understand the risks of travel to Mexico, how best to avoid dangerous situations, and whom to contact if one becomes a crime victim. Common-sense precautions such as visiting only legitimate business and tourist areas during daylight hours, and avoiding areas where prostitution and drug dealing might occur, can help ensure that travel to Mexico is safe and enjoyable."