...like these fancy ladies a few months back at boylston street (i was sitting on the stairs of the public library and they didnt notice me) is what i like most, but what i also find most scary... for me, it also depends whats going on, if there is some kind of gathering, or festivity, i feel more free to take a picture than when its just 1 person, or especially, 1 or 2 cops; they are really too intimidating (at least to me). but, in the US, you are allowed to photograph anything, except: (1) certain military installations or operations. (2) people that are in places that are not easily visible to the general public (ie at home). (i stole this from here). you thus can photograph whatever you can see in public space (thus also police officers, or children, for that matter). it happened however already a few times that we were kindly, or less kindly, asked not to take pictures (for example in the cinema (???), or the area around my lab in charlestown, which is rather industrial and sooo cool!).
here is a link i found very useful, and actually i recently printed the statement posted there and now always have it with me (just in case). found it at this photoblog (also in boston!)