If you're not a Nature subscriber, check out Harvard's website "Running Barefoot or in Minimal Footwear." It has plenty of information: clips of shod-vs-unshod footstrikes, discussions of running biomechanics, and an explanation of how modern running shoes have altered our natural gait. It's a great resource that has just become my go-to link for anyone who wonders a) why I run barefoot, or b) what's up with those funny-looking shoes.
Dr Lieberman was also featured in the PBS series The Human Spark; here is a video of him discussing our species' two-million-year experience with endurance running:
Here is another video of Dr Lieberman, discussing much of what is in his paper: