From what I've seen:
*Harley people wave to other Harley owners if they are from the same demographic. For instance, I've never seen a Harley guy with a new V-Rod being waved to by a guy who has been a "long" time owner.
*Z3 owners seem to acknowledge each other. Other BMW owners don't, such as those with newer 3er, and 5ers the most prevelant BMW's. I guess this is due to people owning the car and not knowing it's heritage, they might as well be driving a Toyota. This is partly BMW's fault for making cars for EVERYBODY.
*Wrangler owners acknowledge one another. I've was in my friend's Jeep with him when he first got it. Another Wrangler owner waved, and my friend who didn't know said "ever since I bought this, other Jeep people wave to me." I explained to him what was up and said "Oh, 'cause this guy I see alot on my way to work waves to me and I thought he was gay."
*Miata owners wave to each other, either with their hand, or by flipping up their headlights.
*Sport bike riders canyon carving wave to each other when they are going in opposite directions.