Jeff Patton has a good blog entry on why he is creating user story maps to maintain the Big Vision.  At the last conference in Boston, I attended his tutorial on user story maps.  Organizing the Product Backlog into a backbone of critical features, a walking skeleton of a barely releasable product and a series of releases is definitely the way to go when dealing with a large collection of user stories.  The best thing I like about the map idea is that it is visual.