When I started on the road, most of the guys I rode with in my working/middle class outer suburb were actually working class Europeans who loved their steel Colnagos, Viners and Pinarellos. Those guys were always friendly and had plenty of community spirit for the younger guys. Everyone waved at riders coming at the other direction, even if we were hammering ourself silly and getting tunnel vision. It's just the way that is was. Cycling was a bit of a fringe activity and we all loved it and had a great time swearing at eachother in Italian.
I've yet to experience this in other cycling realms to the degree that it's present in the road scene. In a lot of ways, the road scene is the worst, because of the consumerist nature of the chi-chi bling-bling scene it's become. It's not just about the bike either - it's wearing the right gear, knowing the right people. Case in point - 'Ride' magazine has the whole back page dedicated to 'what to wear' and 'how to behave'.
I think waving and politeness is a token of cycling solidarity. We're maginalised on the road, we're marginalised off road. We owe it to ourselves to stick together, project a unified front, be courteous, and above all, take time out of your 'serious training session' to wave to a brother.