I went out to the exhibition game last night (the stadium is really nice) and I have some info to report on biking and parking your bike.
Most importantly the bike valet is just off 1st Street SE not N Street SE as the Nationals say it is. This was confusing. Still, you zip right in and there is a fenced off area filled with bike racks to the left (covered and everything). This being a dry run, there was no valet when I got there after the National Anthem. Another cyclist had already cornered a Nationals employee who was on the walkie-talkie trying to figure out where the valet was. After the game there was a valet there and he (pictured below with the receipts) explained that there was a miscommunication but showed me the printed out valet receipts that they'll use. One reason for the exhibition was to work out the kinks for opening night and it appears they have.
I have updated the map somewhat.
WABA worked really hard with the Nationals and Tommy Wells to get the stadium
to a bike friendly point and from what I saw they have. Kudos to them all. (Though, do the police have to park in the bike lane?)