Here's a video from Mikael-Colville Andersen on why we should not wear helmets to bike. I am a firm believer that you should wear a helmet to ride. While there is certainly a sizable minority that disagrees with me, most folks, including potential jurors, feel that the prudent course is to wear one. Certainly, no one will hold it against you if you do.
The bottom line is this: Riding in the city is dangerous, period. So wear a helmet. However, it is not so dangerous that you should avoid riding all together. The middle path people; take the middle path.
The video is via Momentum Magazine, via Urban Velo.