What I heard at childbirth class last week was that diarrhea was a sign of labor. So, if you have diarrhea it could potentially be that you are getting real close. After all, people usually have to go to the bathroom multiple times when they have loose stool.
Like everything else at the end of pregnancy, It can mean something or nothing at all. I've been experiencing this for about 3 weeks and I have 3 weeks still to go.