I always smile or wave or whatnot at people when I'm out, both bikers and runners. Anybody out on the road who's actually expending energy to make themselves healthier or save a little pollution is fine by me. Sometimes hardcore roadies will ignore my friendly (and therefore fredly) behavior, but most people are pretty cool.
I did just about give some lady a heart attack this morning when I said "on the left" when passing her. I didn't notice her (head down climbing) until I was about 10 feet from her, so I think I just startled her. She just about dived over the guardrail.