Yeah this game is coming out sometime this year I believe. Anyway the story is something along the lines of "its a Authoritarian government and when people need info sent, they give it to special runners that evade cops and such by any means, then something happens involving your sister and the game starts."
Anyways, the entire game is all about free running and such and looks sick. If you liked Assassins Creed AT ALL, you seriously gotta check this out.
This video is all in game footage, which makes it even more amazing, I can't really explain it.
Gametrailers.com - Mirror's Edge - E3 2008: Leap of Faith Trailer HD