Virtually all serious and semi-serious cyclists here (Reno) wear them. You see the occasional wannabe racer who's too cool, and many once-a-month riders wobbling along on Huffys and stuff without them, but nearly every "real" rider on a "real" bike will have a helmet on.
What bothers me are the number of kids you see riding with parents, and the parents will be helmetless while the kids have on token plastic, but tilted way back so it won't do any good when they faceplant. I don't know how you can NOT think of that when you look at the kids.