Now here's the official word from Metro (link to original:
http://www.wmata.com/about/MET_NEWS/PressReleaseDetail.cfm?ReleaseID=2085) - here's the part about how the Nationals matchdays will be handled:
Washington Nationals Games
On May 9, 10, 11, and June 6, 7 and 8, the Washington Nationals have home games at Nationals Park. People attending these games should add more time into their schedules if riding or connecting to the Yellow and Green Line as there will be longer waits for trains before and after the games.
Travel options for riders before the May 10, 11, June 7 and 8 games:
Metro will operate shuttle trains between the L’Enfant Plaza and Navy Yard Metrorail stations on the Green Line.
Red Line riders from Shady Grove who transfer at the Gallery Pl-Chinatown Metrorail station should transfer at the Metro Center Metrorail station and catch a Blue or Orange Line train to L’Enfant Plaza. From there, they can board a Green Line train to Branch Avenue or a special shuttle train to the Navy Yard Metrorail station.
Fans on the Red Line can travel to the Union Station Metrorail station and Blue or Orange Line fans can travel to the Eastern Market Metrorail station and catch the N22 (Navy Yard shuttle) Metrobus route that provides direct service to the Navy Yard Metrorail station east entrance at New Jersey Avenue, SE.
Travel options for riders after the May 10, 11, June 7 and 8 games:
Metro will operate shuttle trains between the Navy Yard and L’Enfant Plaza Metrorail stations on the Green Line.
At the end of the seventh inning, Metro will operate free shuttle buses every five minutes between the Navy Yard and Federal Center SW Metrorail stations. The shuttle buses will depart from M Street and New Jersey Avenue, SE.
At the end of the seventh inning, Metro will operate free shuttle buses every five minutes on the N22 (Navy Yard shuttle) Metrobus route from Navy Yard to the Eastern Market and Union Station Metrorail stations.
Extra station managers, rail supervisors and Transit Police officers will be at the L’Enfant Plaza, Gallery Pl-Chinatown and Navy Yard Metrorail station to assist riders and to direct fans to the free shuttle buses after the baseball games. Metro will also have volunteers available to answer questions from riders on location of shuttle buses and how to walk to the nearest station and making sure fans have enough money on the cards to enter or exit the Metrorail system.
To alert customers about this project, station and train announcements will be made throughout the Metrorail system. Additional information will be available on Metro’s Web site.
The Trip Planner will not reflect the impact of this work on its schedules. Customers should allow more time for their trips because of this work. For more information on traveling by Metrobus or Metrorail, customers may call (202) 637-7000 or TTY (202) 638-3780.