Once, back in early 2008, while down in London Oxford St. I saw this guy who was using what seemed like a Nikon D3 and the 14-24 AF-S (both were very recently released). He was walking at a relatively fast pace, especially considering the busyness of Oxford St., and was getting right up there in people's face taking shots.
I envied him for his kit, his courage to get up there (this is a full frame, so even 24mm is way too wide).
I tried it, on another visit (D700 and 24-70AF-S kept at the 24mm mark) and you REALLY have to get close to people. It was then that I realised why he was walking at a fast pace, people do get a bit, tiny bit, annoyed at such close invasion of their personal space (mind you that guy I saw was REALLY handsome .. may have helped him a bit :shrug:

)
Personally, I do a lot of this type of photography, but only where it "seems" to be socially and morally acceptable (busy cities, busy parks etc.). My lens of choice is in the 58/1.4 Voigtlander, or the Nikon 50/1.8 .. but there are times when I've used my 70-200VR or even the 70-300VR. From all that, I've found two things intimidate people more than anything, getting too close, and using a big lens. With smallish primes people are not too weary of you pointing the camera in their face, and most are happy of you to shoot them, in a way they "seem" flattered; but not so much with the other two approaches.
But, as Chris (from above) has said .. does it really matter what focal length you use? Your aim is to capture a specific shot that gives a specific story. May be being too close ruins that, may be being too far ruins that ... may be getting the subject to notice you ruins it, may be not .... it is all down to what you want to capture.