As a cyclist and well trained driver (more licenses and drivers training/saftey courses then average, in the USA at least) I don't think helmets will help/hurt much.
I don't wear a helmet except for events/races(lots of close cyclist make me nervous, and odds of an accident likely go up) and while I'm sure a helmet would help if a car hit a cyclist, I'm betting it'll cause lots of damage, helmet or no. No, the problem in my town is it seems cars *try* to cause bike wrecks. Shouting, tossing things at me and my two wheels, or just not watching for cars, never mind less visible bicycles come to mind for reasons why. It doesn't matter that I try to avoid cars, do everything to stay out of busy traffic, drivers still do those very annoying, dangerous things.
That's not to say I haven't done stupid things with cars and bikes, I make mistakes like everyone. But if drivers and cyclists try for safety and to share the roads, it'll help alot more then helmets ever could.
(*I'm sure there's speeds/circumstances/accidents where helmets make all the difference, but it's likely a very tiny percent.)