I was just thinking about this today, actually. I failed to wave at another biker while I was changing lanes. I was checking over my shoulder and basically missed him/her. I felt a little guilty. But more to the point; I always make an effort to say hey. If it is a bike or rider I know, I tend to throw "the horns". If its an amazing machine of one sort or another, the wave is accompanied by a full seat-twist to "take it all in". If its the first day of sun (I live in Seattle) I have been known to throw both arms up in a "hoooray" sort of fashion.
I firmly believe that the wave is an affirmation of character and spirit...its why motorcyclists are some of the most interesting people a person could meet. Hell, I even wave to scooters from time to time...those SOBs are nuts to ride those under-powered squirrely little things year round in the city. So maybe I get my milk money stolen from all the "cool" kids at school, but two wheels is two wheels.
And, for the record, Harleys never wave back. So F*** them. But to be fair, I giggle a little every time I see a Hog low-side.