An article in today's New York Times (http://www.nytimes.com/2009/10/27/health/27well.html) is about the growing body of evidence that the simpler and more basic the trail running shoe, the better it is for you. The article talks about evolutionary and anthropological evidence for the proposition that humans are well-suited to run great distances.
The article also talks about why so many distance runners today are prone to injury and mentions three likely reasons: (1) distance running tends to be popular among older runners, many of whom did not start running until later in life and thus their bodies aren't as prepared; (2) training programs that are too aggressive ("run a marathon in only 16 weeks!"; and (3) running shoes that provide too much cushioning and support, which, over time, negatively affects the ancillary muscles, tendons, and ligaments needed for a proper gait, thus the shoes themselves contribute to injury.
The lessons to take from this? Don't overtrain, don't try to run through injuries, train on dirt/grass trails where possible instead of hard pavement, and seriously rethink the shoes you're using and move towards simpler, unsupported, minimally cushioned shoes.
My editorial comment is that this dovetails perfectly with Kenyan and other contemporary east African distance training programs (along with those now being used by top American runners such as Kerri Wells and Ryan Hall, to name but a few). I read that even Texas Tech football coach Mike Leach has installed a sand track and he requires players to run barefoot on it, believing that the unevenness quickly develops greater leg stability, thus reducing injury.