There has been increased violence in Mexico due to the drug war, but most of the violence has taken place in specific areas, mainly along Mexico's border with the United States. The great majority of cases of violence, like the murder of the general who was involved in the war against drugs, have been between Mexican authorities and the drug cartels. So while you should take the travel alert seriously, and always practice general safety precautions, there's really no reason to avoid travel to Mexico completely. Mexico is a big country and there are plenty of great destinations where you can have a relaxing and fun vacation, and that includes Cancun.
Expat bloggers living in Cancun weigh in on the subject:spring break safety tips and have fun!