An animated trailer from DICE and EA reveals a little more on the plot in Mirror's Edge.

Mirrors Edge - trailer (story)

In a city where information is heavily monitored, agile couriers called Runners transport sensitive data away from prying eyes. In this seemingly utopian paradise, a crime has been committed and now you are being hunted. You are a Runner called Faith - and this innovative first-person action-adventure is your story. Mirror's Edge™ delivers you straight into the shoes of this unique heroine as she traverses the vertigo-inducing cityscape, engaging in intense combat and fast paced chases. With a never before seen sense of movement and perspective, you will be drawn into Faith's world. A world that is visceral, immediate, and very dangerous. Live or die? Soar or plummet? One thing is certain, in this city, you will learn how to run.

Mirrors Edge - gameplay (E3 2008)