I remember quite recently reading about a couple cyclists who were attacked in southern Mexico. This, I think, has given rise to a few more reports or scare mongering but how many are urban legends and how many actual events is not clear. I can't remember the website of the cyclists who were attacked right now -- sorry -- but I think they were raising money for diabetes and I think I saw the link to their site on the Lonely Planet's On Your Bike forum. You might do a search. Best regards.