I was thinking about this thread on a ride the other day. I wanted to see what kind of rider gave me the most response. It was early in the morning and the crowds were not out yet, so it was a small sampling. I passed only a hundred-or-so cyclists on a 25-mile ride and responses ranged from a nod, a smile, a fingers-off-the-hood-wave and a hearty "good morning!" but here are the results of my unofficial tally:

• Male roadies (relaxed pace): 60-70% said hi first or responded*
*Note: older roadies 30+ responded more often than younger ones
• Female roadies (relaxed pace): Only 3, but 2 responded.
• Roadies (in obvious training or paceline): Didn't bother, either way

• Male hybrid/MTBers/commuter: None said hi first, about 10% responded
• Female hybrid/MTBers/commuter: None said hi first, about half responded

• Male recreational bikers: None said hi first, about half responded
• Female recreational bikers: None said hi first, about half responded
Note: some were obvious tourists and may not have spoken English. Riders with kids responded most often.

To clarify, I was a 30-something 105-lb gal riding alone, on a Specialized road bike in full Castelli gear. I think roadies are just more accustomed to respond to their "own". I've even been told (jokingly, I think) by a couple of roadie pals, "You're one of us now."
Had I been on a hybrid, or if I had been wearing my Nellie Olson Rocks! sweatshirt, I'm sure my results would have been different. But that test is for another day...