Past Horizons recently posted this illuminating article on vandalism in the Deer Rock Art Center in Arizona, which has the largest concentration of Native American petroglyphs in the Phoenix Valley, dating between 500 to 7000 years ago. Not only are historical sites in the area suffering defacement, but looters are hacking out chunks of rock art and "dislodging anything they can carry away". There will be a free panel discussion Saturday, February 5, at the Deer Valley Rock Art Center to talk about how the sites can be protected by teaching the public to value them. If you're in the area, you should seriously consider going.
The article also provides this Flickr site of rock art vandalism. Horrifying and enlightening.