I spent the first 23 years of my life living in places where greeting other runners is standard (e.g. a nod, smile, "good morning"). But I live in Southern California now, and it's hard--sometimes impossible--to get other runners to acknowledge me, even when I say hi to them first. Other people I run with who are from this area think I'm weird to greet strangers on trails, which makes me think this is a regional thing.
Where do you live and is it standard to greet other runners while you are running?