One thing that nobody's mentioned is that the behaviour is very self-reinforcing, and actually says a lot more about the environment than the individual. The same person who goes for a run through a park in London and doesn't acknowledge anyone may well find themselves on a run in the countryside one day, saying hello to everyone. You get used to the way things are done. We're social animals so we mirror other people's behaviour.
When I started running I naively estimated (from my earlier cycling days) that members of this little community would acknowledge each other all the time (not necessarily anything as daringly ostentatious as an actual "hello" or cheery wave, just a subtle nod, smile, or even the raising of an eyebrow that only the keenest auctioneer might spot) but after a short while you work out that roughly one in ten people will reciprocate. So from then on you sort of flit between (a) being ready to catch someone's eye and reciprocate any inkling of an acknowledgement, being careful not to be blanked completely and therefore made to look like a fool, and (b) staring straight ahead to avoid any such social embarrassment altogether.
The odds of receiving an acknowledgement will vary even within the urban environment, depending on factors such as the weather (as already mentioned; shite-awful weather = knowing nods and smiles from smaller, select band of runners to acknowledge our shared hardness/smugness), environs (the high street compared to a canal path strewn with swans and geese), the amount of time between first spotting the oncoming runner and passing them (more time to make up your mind whether to adopt strategies (a) or (b) above), etc., or your mood at the time.
Not that I've thought about it or anything.
Edited: 21/12/2011 at 11:39