The only problem I have ever had with photographing people without permission was in Jamaica. There are lots of interesting people there to photograph, but several times I have been approached by a person demanding money, since I just photographed them. Maybe its just the culture there that they don't want their picture taken without permmission.
I would also be very careful not to thake photographs of children without the parent's permission.
This also happens frequently in China. People of lower economic status often run to tourists to allow the tourists to take pictures of them, especially with their animals like water buffalo, of course for a price. While on a tour we had a woman who looked to be 80 and so skinny I was amazed she could even walk, waddle up to us as fast as she could from about a football field away dragging along this water buffalo which she was taking out to graze. Of course she wanted payment for it, which we did. We gave her 5q. which was all that we had left (about 80american cents) and she looked at us as if we were the most generous people in the world. Its amazing how little those people can make. So thats just my little chinese story about payment for pictures. Another great picture from that trip was of a buddhist monk, in a huge buddhist monastery wearing white gym shoes with a pink playboy bunny on them.
I didn't have any troubles with confiscating cameras on that trip. I didn't have a camera of my own and all pictures were taken with point and shoots. however I hope to go back to China this summer. I plan on taking as much wildlife photography as I can, but I am also very interested in taking pictures of the people, emphasizing cultural differences and their government dealings. Anyone have any tips? I will finally have my own camera (a D90) so I really really want to protect it.