This is a source of much heated debate. I'm not qualified to evaluate the various claims, but here is a quick summary of some of the issues that are used to argue against helmet use:
- Helmets may cause increased neck or spinal injuries because the large bulky shape of the helmet will twist the head more than a bare head will. Especially a problem with the race bike type "aero" helmet with a big extension on the back.
- Helmets may cause riders to ride more aggressively because they feel more protected. This may result in more injuries overall.
- Helmets can cause a head impact where one would have been avoided by tucking the head and rolling because the head is larger with the helmet on.
- There was one study that showed that cars passing a cyclist will pass closer to one with a helmet than one without. However, the assumption that this means helmeted riders will get hit more often is highly debatable.

A seperate issue is the argument that wearing a helmet implies that cycling is an inherently dangerous activity and that this will discourage other riders. It has been widely documented that there is very good correlation (and likely causation) between the number of cyclists on the road and the number of accidents. Many cities have seen overall bike/car crashes decline even as the number of cyclists climb. This leads to a separate debate that often gets mixed in regarding helment laws.

One thing most people agree on is that helmet laws discourage cycling. And while the remaining riders might be less likely to die in an accident once one occurs, they are more likely to be in a accident due to the decrease in cyclists overall.

If you really want to look into the debate some, you can visit this to start:
http://www.bikeforums.net/showthread.php?540354-Helmets-cramp-my-style-Part-2&highlight=helmets+cramp+my+style

A warning that arguments on this subject tend to get very dogmatic.

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