Bike helmets are only designed to protect you from the impact of losing balance and falling over. They are not designed or medically recognized as offering any protection from crashes whatsoever and are only specifically suggested/required when cycling over walking or running because the public would never agree to wear them while walking or running.

Neurosurgeon Dr. Michael Schwartz said the following in his expert testimony which led to the passing of a mandatory helmet law in Ontario:
"I have no concrete suggestions for improving helmets. It is always a compromise. You could make a helmet that would be far more efficient or far more likely to cushion the impact, but it would be bigger and heavier, and if you made it big enough and heavy enough nobody would wear it. There is always a tradeoff between size and convenience and effectiveness. Right now we are at some sort of level of convenience that still provides protection."
"There is excellent evidence in the medical literature that shows helmets will mitigate the effects of falling off your bicycle and striking your head. They are designed to reduce the G-force administered to the brain when the head strikes the ground and they are likely effective if the person falls from the height the head is at when a person is cycling. If a cyclist is accelerated by a car, swept up on the hood of the vehicle, to a speed of, say, 40 or 50 kilometres per hour, then the helmet will not work and will not prevent a severe or even fatal head injury. So I think everybody should wear helmets but should have a realistic expectation about what they can or cannot do."

Considering the epidemic of heart disease in America your decision to cycle is perhaps the safest choice you will ever make, but be careful. There's no shortcut for gaining experience and, unlike the steel cage interceding in your first car accident, an inch of styrofoam isn't going to do beans for your first bike accident.

That being said I personally wear a helmet (Bell Metro) sometimes, but not always, because I think it's better than a hat for mounting lights, earmuffs, etc and less likely to get caught in a gust of wind. I also believe that if you are a new biker then you should wear one because you are more likely to fall over until you have more experience balancing on two wheels.