I called someone in the Admissions Office at Michigan and she said that she put a note in my file that I inquired about my application and that I was interested in coming (which I definitely was).
I then wrote a letter to one of the professors basically saying that Michigan I want to go there and specifically how I would use a Michigan education to help the local Detroit/Michigan community (so basically, make a case as to why I had to go to Michigan).
It definitely helped me. I don't have as great of stats as some of the other posters here, but I just got accepted to the University of Michigan for Fall 2010.
My 2 cents: Calling won't hurt and really making your case won't hurt. If you don't get in, a call wasn't really what ultimately held you back AND if you play your cards well, you might actually make something great happen!