I went for a long ride yesterday and saw a couple of people without helmets; the vast majority had helmets on, though. But this vast majority that I refer to was on rental bikes, too.

But I saw some helmetless riders who seemed to know what they were doing. Could it have anything to do with the fixed gear trend? Seems pretty rare to see someone on a fixed-gear bike wearing a helmet. I live in San Francisco; perhaps it's different in other places.