I have been to Mexico for the past 3 years and have never had any issues. But common sense should prevail.
Last February my girlfriend & I spent a week in Mexico City just walking around shooting local interests. Both of us carried Nikon D80's and while the odd person asked us for money, we didn't feel any threats. The people are extremely friendly and curious and everyone has a friend who take you somewhere, for a price. We even had a police officer give us his card as when he is not working, offers guide services.
The National Place is a must and a day trip to the pyramids is worth the ride. Mexico City is a big, vibrant city and learning a little Spanish will go a long way to your enjoyment. Here is a sample of some of the photographs I took while I was down there last year.
http://gallery.me.com/hmpost#100032For the past 3 years I have spent 3 - 4 weeks in a little town 3 hours north of Mexico City called San Miguel de Allende. Paul Rimstead used to spend his winters there writing for the Toronto Sun. Toller Cranston has a villa there as well. A lot of Canadians and Americans call the place home and English is a common language there. Great photographic opportunities. Santa Fe Workshops holds numerous workshops in San Miguel and they produced a info pdf talking about the safety in Mexico. I have attached it.
Mexico can be very enjoyable and presents a lot of great photography opportunities. I feel as safe there as I do in Toronto, but saying that, you won't find me down on Queen and Parliament in the dark.