2000 N St NW accepts dogs (not sure if there is a limit on size, there may be). Its called The Flats and is in south Dupont (a few blocks from Farragut North and Farragut West (red line) metro stops). All of my friends that live in DC that have dogs live in Dupont.
Anyone summering in DC with a dog? I am, and am trying to find a roommate with the same situation.