I've had the expereince as the other posts above. I get more returned waves out of sport bike riders on the average to include the head nods if busy. I always pass a few of the 'real cool' harley riders who won't acknowledge you period. That said, I've also picked up behind two different GOld Wing riders recently who didn't return anyones wave, didn't acknowlege my presence behind them and were all over the lane, so I just kept my distance and let them ride.
Each to their own I guess, but I'll continue to acknowledge all modped and up