Where I live in Northern VA, local ordinances (i.e., County govt.) dictates a helmet laws for mopeds, while the State Law doesn't, only for motorcycles. Bicycle helmets won't pass the muster as far as the local police are concerned here. Its a motorcycle type of helmet or a stiff fine. However, I think Jaime and Doug are on the right track and I agree with them. I always wear mine. It's basically "cheap insurance". Also, God forbid that you do get into an accident, the insurance laws here severely limit your recovery ($$) w/o a helmet, even if it is some other clowns fault.
About a month ago a Dude in my area tried for a Moped Darwin Award. Early in the evening he stole a Tomos and was cruising on it @ 5:00 AM when spotted by the local police. According the the newspapers accounts w/pic, the local police proceeded to stop him during a "low speed chase". The Dude had a helmet on, though unbuckled (I guess he thought is wasn't kool to buckle up). He failed to make a right turn onto a dual lane street, fell off the ped, losing his helmet on the initial impact, then fatally hitting his head on the opposite curb. End of story - - DOA. I've personally driven my ped & scooter, through the intersection. In my estimation, the Dude would have survivied easily w/a helmet, maybe a broken arm or leg but nothing in the fatal column. Rich