The blog Improbable Research has profiled Daniel Lieberman (Harvard professor of evolutionary biology, featured in Born to Run, and incidentally winner of a 2009 Ig Nobel Prize for his discovery of 'why pregnant women don't tip over').
The post includes a 6 minute video with some very interesting super slow mo video and force graphs demonstrating the difference between heel-striking (Bowerman) and mid-foot/forefoot striking (which we almost impulsively do when we don't have cushy heels on).
Doing all your running without shoes may not be the best idea (especially in the cold, wet, snow, etc.), but I certainly believe it is worth doing once in a while. In my opinion, occasional barefoot running can really help to remind you how to keep a healthy form that reduces injury.