A friend e-mailed me this interesting article that takes a pretty extensive look at all the major helmets on the market. It's quite long, but is a very interesting read, especially when you look into some of the more expensive helmets on the market which may not necessarily provide better protection than some of the cheaper models. Couple of items I found interesting - get the helmet with the least number of vents you can bear, avoid points, and helmets with harder foam tend to protect less in slow speed impacts.

http://www.helmets.org/helmet09.htm

Hope this helps.