Here's a link I found a while back that explains the whole wave / don't wave thing... I'm not sure when all this started but it seems to me, back in the day I never saw the epidemic like I do now. It's especially rampant here in CO, I wear my arm out on mountain rides on the weekends when all the pampered riders emerge from their dens to embrace the sunshine! hehehe
Anyway... for your pleasure, here's the skinny...
http://www.viewfromthecloud.com/2006/08/secret-motorcycle-hand-greetings.html
Ya'll keep safe out there!
The Samurai