I'm from Boston. I saw spontaneous group conversations break out among strangers fairly often. Certainly, I would not think twice if it happened there. In eight years in Seattle, I have never seen it happen, not even once.
Being from the midwest, I've taken the train in Chicago many times, without ever having an experience like I did in DC. I would say that commuters in Chicago tend to be comfortable exchanging glances with one another and making smalltalk, quite unlike DC.
I have the opposite second-hand report, though: a friend from California tried introducing himself to his new neighbors in Boston and basically got no response, as if introducing yourself to your neighbors was a weird thing to do. His general impression is that Bostonians tend to socialize, but grouped along very specific location/class lines, where you're a "foreigner" if you're from even as far as Somerville, let alone California.
Coffee shops and dog parks on the other hand are prime stranger conversation spots. Oddly not bars though in my experience. I'll be moving to Cambridge in a few months so it's good to hear people there are more approachable.
One small contrary datapoint: on my first visit to Seattle from Boston where I was living, I was walking on a downtown city street, uncrowded at the time, and an ordinary passerby made eye contact and said "hi". I almost stopped in my tracks, that interaction would have never happened in Boston at the time. There, it seemed that all strangers in public were to be perceived as threats (probably beggars) unless there was evidence to the contrary. This sort of (perceived) general friendliness was partially what motivated me to move there, that along with the mountains+water and being able to drive 20 minutes from downtown and being in the country. Yeah, Seattle folks, it really was like that on the highways once, not too terribly long ago.