Learnt about this guy on the Australian Photoholics forum ... he has some serious balls, he uses a 50mm and a 20mm lens and basically sees interesting people and walks up takes 2 or 3 photos as they look at him then he walks away again ! He says that it is very rare that anyone will question him and has not had any trouble doing this (I find this hard to imagine) but I guess by the time you look up, register that he is pointing a camera at you and taking photos, think "this guy is taking a photo of me, I should say something", think of something to say, he is gone again.
I cant imagine myself doing that, but I do like some of his stuff.
http://500px.com/85mm