i'm looking for linkable info, but someone did a study about helmets in traffic.
his findings were that drivers were more aggressive towards those wearing helmets, and typically gave them less room than those who didn't (he took a "wear a helmet for x days, then wear no helmet for x days approach; same people, same bike, same roads).
his conclusion was that drivers view those with helmets as less threatening, and were subsequently less likely to feel obligated to respect the drivers (wearing a helmet also increased verbal confrontation from drivers to cyclists).

like i said, i'll look for this guy's stuff so i can link.