Dr. Daniel Ferris a professor of movement science (where was that major in college?) at the University of Michigan's School of Kinesiology.  Ferris states that even though the common thought is that grass and dirt will lead to decreased injuries, the data actually doesn't support this theory.

This article highlights a research paper released in 2008 by Foot and Ankle International, which revealed that running on asphalt vs. running on sand leads to the same amount of overuse and strain injuries.

Well here is where the wheels fall off this article.  It is not detailed whether this paper looked at proper footwear, previous injuries, level of runner, etc...  Also, comparing running on asphalt to running on sand is completely erroneous side-by-side as anyone who has run on a loose sand beach can tell you.  David Hasselhoff made it look easy, but running on loose sand is a tough workout, but does not lead to normal running mechanics.

Personally, I suffered from medial tibial stress syndrome or shin splints for almost a year, until I started to incorporate proper mobilization drills, stretching, and switching from pavement to trail running.  Once I hit the trails I never looked back, and I've NEVER suffered from shin splints again!  Is the trail the answer for everyone, well no, but I do think that it should make up a portion of every runner's regiment.

In this article Dr. Bob Adams, Chair of the USA Track and Field Sports Medicine and Science Committee states that "the ideal would be to run some of the pavement, some on the trail and some on the track".  I think this is a good plan, but not always possible depending on where you live and time constraints.

I really believe that with proper running technique and warm-up you should be able to run injury-free, so long as you train smart, which means not increasing mileage too quickly, no cross-training, choosing the wrong footwear, proper nutrition, and adequate rest.

For those of you who do suffer from MTSS (shin splints), here is the gold mobilization drill that I personally do pre and post-run.
 

Now, this mobilization is not a magic bullet, but being diligent with this and adding in ice massage, proper footwear, possibly some rest, cross-training and maybe even compression and some Kinesio tape is a powerful combination for shin splints.

Here is a link to the article
http://www.outsideonline.com/fitness/fitness-coach/The-Best-Running-Surface-for-Your-Knees.html