I carry a camera around alot.
They have adds about things like that down toward NYC, they also more disturbingly have adds about reporting people who look nervous, out of place or avoid eye contact.
Yep....I'm fucked.
I never got hassled over my camera yet, apart from a few strangers who didn't take kindly to getting their photos taken. Fair enough, some people will be touchy regardless. Still nervous when doing street photography though. I usually ask people to be polite, but sometimes catching them unaware yields more natural, interesting results. Shows the human condition off more, IMO.
Here's an example, these two were completely unaware:
If I went up and asked them, I most likely will not have got such a result. People stop being natural when they're aware of any observation.