On a recent Saturday evening, about a hundred serious bicyclists, most of them young men, many tattooed and pierced and at least one wearing striped tights and a floral thrift-shop dress, arrived en masse at Alberta Park in northeast Portland, Ore. They gathered near a fenced-off hard-top court and, in teams of three, began a ''bike polo'' tournament. Almost all were bike messengers, about a third of them local (others from Seattle, San Francisco and elsewhere), and they lived up to the image of couriers as marginal, testosterone-charged troublemakers. They drank beer, smoked cigarettes and other things and yelled profane...