There is (and generally has always been) political unrest in some of the southern states in Mexico. Tourists and foreigners are not the target though, if you remember the Coup of San Christobal de las Casas some years ago, tourists were getting their pictures taken with the rebels!!

Oaxaca, i believe is still under "popular control" and under siege by the state - shame because I was there in 2005 and it is one b-e-a-utiful city.

As with any developing country there is always a slight risk if you venture off into the wilderness, though generally the archealogical sites are designated tourist destinations and safe. The Lonley Planet guide book is generally pretty good for Mexico, and gives clear warning of any places or roads where tourists have experienced trouble. Geneally though, these tend to be in the north, boarder towns, and the cannabis growing lands in Jalisco state, and maybe you don't want to wander off the beaten path near the Guatamalan boarder as it is believed that a large amount of USA bound drugs go this way.

That being said, I travelled for 3 months, solo, through rural Guatamala, Belize and Mexico with no problems except a dose of the ..... (And that was in a resort!!) Just try to to look and act like a Gringo haha..

Edit: If you can speak a few key phrases in Spanish, this will help you immensely! And remember, EVERYTHING is negotiable in Mexico, and it usually happens tomorrow!