is this an america only thing? I've been all over the world (except america) and never ever felt remotely concerned about being robbed for my camera gear, obviously i keep it close at ahand and dont leave it lying around for opportunistic thieves but as for being assaulted and robbed? no way
Just like everywhere else, America has its bad spots, some worse than others, and the few that you just dont want to ever go near. As far as the "bad" places, most of the people that would attempt to do you harm wouldnt know the difference between a disposable and a dslr, they would just take it from you either way. For the most part though they are going to leave you alone if you leave them alone. Just remember, safety in numbers so bring a friend or two and let someone who isnt there know where you are and when you expect to return...
Oh yeah, and dont be flashy with your gear or clothing either...