I'd say that approx. 98% of roadies I pass in the opposite direction wave back, or wave at the same time that I do. I usually wave first. However, if flying down a fast hill while passing riders going up, to keep my hands on the bar, I always yelp some verbal greeting. At the minimum, I will at least give a head nod.
It really is pathetic for a fellow roadie not to wave back, verbal greeting, or at least a head nod when he/she clearly knows that hes/she has been waved at by another cyclist. Life it too short to behave otherwise; some of you much younger than 50 will realize that later in life, hopefully not too later.