Here at the beach, it seems to depend on the camera. 35mm draws very little, if any attention - lots of tourists have them. When I'm using the 4x5 Speed Graphic, people get out of my way and stand back a respectful distance, cops and firefighters let me get close to the action. Folks must still associate them with Serious Newspaper Work.
The 8x10 attracts the nuts. In a public area, they seem to wait until I'm under the darkcloth to walk up, tap me on the arm, and say, "Excuse me. Excuse me. What are you doing?" Before I learned better, I tried to engage these interrupters in polite conversation. Now I use my own version of Francesco's trick - I come out from under the dark cloth waving my arms and shouting "I don't speak English" in Serbo-Croatian. That usually frightens them away. (I'm working on Mandarin Chinese as a backup.)
One woman was persistent. I finally frightened her away by saying, "Listen lady. I'm a lawyer. If you don't leave me alone, I'm going to sue your ass off." That got rid of her.
I think I'll try Aggie's way next.