I think all of this arguing about how to greet (or not greet) other riders is probably location-specific. I've lived in an area that was thick with cyclists -- Colorado's Front Range. On a sunny Saturday, I might see several dozen other cyclists on a 50-mile ride. I still think a nod or a slight wave is polite, but not everyone does it, and there were probably times when I didn't either.

Since then, I've lived in more rural areas -- where, on the rare occasion I spot another cyclist, I will chase him/her down (or slow down, whatever) to introduce myself and chat a bit. I think it's happened four times in the past three years.