I've lived and run in many cities, and I'm always a keen observer of whether runners in an area give you and acknowledgement via the hand wave or head nod. I used to think that is was a small town vs big city thing, but I'm not so sure. the worst culprit is the slow female running w/ a walkman, too much make-up for a morning run, and probably running less than 2 miles. my all time greatest nod was a nod from Steve Jones while he was running along the Charles Rivers a couple days before boston.