Found this article while I should have been studying for a test.
http://www.sportsscientists.com/2008/04/running-technique-footstrike.html Very interesting, and I think the conclusion the author draws is probably correct. I've tried to consciously change my foot strike before, with no permanent results (I don't remember if it resulted in injury or not). I have found (duh) that the faster and more efficient I become, both in general and on individual runs, the more my strike tends to move towards the mid or forefoot, but when I go on a slow run I immediately revert to heel striking. Anyway, the article is interesting. I wonder if there are more studies than the one cited by the author.