Money has been approved from congress for the Silver Line towards Dulles Airport- for the first segment to Tyson's Corner. This will provide easier access to the supermall there, and should spur development. Some groups even say that the area will become DC's skyscraper district. (I know it's Fairfax; Rosslyn is unsuitable because of the 300' height limit due to National Airport, and the feds probably don't want a building towering over the Pentagon, if located in Crystal City.
But will it be above or underground? It still has to be decided. Even if it were overhead, the cheaper alternative, the architectural drawings do make it look not like an eyesore.