I normally try to catch other runners' eyes, and smile and nod to say hello if they look over. Most run past without even glancing my way. Doesn't really bothers if they don't tho.
I did once entertain myself on an 18m run round Richmond park by keeping track of how many runners I could get to say hello. I reached a count of 30 or so but it was only about 10% of the runners I passed.
I do think it's sad however that I wouldn't get a hello from muttley as I wear an iPod. Maybe I should experiment and see if I get more hellos if I leave it at home.
Finally I think it is partly a city thing, I found I never got acknowledged by runners when I ran on the Brighton sea front, but did if I went on the downs or on the cliff tops beyond Brighton marina (including once being high fived by a runner!)