I actually just went out last night in Baltimore with three friends to photograph some neighborhoods and, at least with old film cameras, people are usually just curious. I get "Ya'll filmin' a movie?" or questions about what I'm doing a lot, but I think sometimes people don't know the value of the equipment we're using because it's not a big expensive-looking DSLR. At least in the inner city, people are genuinely interested because it's something different. I've actually had more problems with the suburbs then I've ever had with the city. I've never had an incident where someone wanted to rob me for my stuff (knock on wood), but you definitely need to be cautious. I would definitely have a buddy or two, just in case.
Shooting photos at night is the best.