i think you'll find that if you whip out a 600mm in a neighborhood (where you don't really talk or know your neighbors) like mine, you will get a reaction. the first few times i pulled mine out blinds started going down. it was hard not to laugh. over the last month or so they've realized that i'm just shooting the sky with it. i still don't bring it out in the daytime.
overall i think it's impossible to give a blanket response to this because of all the different types of photos one can take of people. i find the napalm girl and interesting case. mentally i wrote that off as "oh, that's fine. it's news". i completely discounted her as a person. i only noticed that i did so when i thought about how many times i watch the news and have thought "if someone ever were to harm my family and some POS cameraman from channel 4 news showed up and just started shooting the scene without asking i'd belt the guy". it's a complete violation. why? because the subject thinks it is. the feelings of us camera people is irrelevant. remember, the topic here wasn't legally framed, it was ethical in nature. the interesting thing is that ethics wasn't defined. whose ethics? ethics of photographers or humanity? i prefer to follow what i believe to be proper behavior of a person first and as a shutterbug second. on the flip side i would lump in the papparazi who generally act in the reverse.
hmmmm...rambling. i guess my point is that i'm in the school of thought that believes you owe your subjects the respect enough to be given the opportunity to decline. i think post-exposure permission is fine.