The Washington team website is up...
http://mlb.mlb.com/NASApp/mlb/index.jsp?c_id=was
They will be using the old cursive Senators II "W" on the caps, blue cap for home, red for road. "Nationals" on the home uni, "Washington" on the roads. Also a "DC" (looking a lot like San Diego's "SD") for some secondary uses.