I have ridden motorcycles since the mid seventies and acknowledging an other motorcycle rider with either a wave or a slight head movement has always been done here in Australia as far as I am aware. Of course Hardley riders only nod at other Hardley riders and BMW riders are too scared to let go of the handlebars. Apart from that it is just courtesy unless you are just too cool.
By wave I mean just a slight hand raise not wildly waving your hand like a d1khead.
