Washington, D.C.

In the early 21st century, when the United States was divided into thirteen commonwealths, Washington, DC, the pre-War capital city of the U.S. became part of the Columbia Commonwealth, along with Virginia and Maryland.

Pentagon/Citadel
The Pentagon was largely destroyed during the Great War. But its sub-levels remained intact, and contained various pre-war technology and weaponry. When the Brotherhood of Steel arrived at Washington, DC, once the defense robots were destroyed, started to retreive the technology from the site. But there was something else, as well… a technological marvel that, if restored, could help the Brotherhood rebuild a strength and reputation that had been declining steadily for years. Paladin Owyn Lyons, the leader of the Brotherhood group, became the Elder of the Brotherhood's Capital Wasteland division and established a new, permanent Brotherhood base called the Citadel, built into and beneath the ruins of the Pentagon.

Other places
One of the concept art images shows the (largely intact) Capitol, while another shows the Jefferson Memorial converted to a water filtering facility. It should be noted, however, that those two images by Craig Mullins are early concept art and do not necessarily reflect the actual game.

Appearances
Washington, DC will be one of the major locations in Fallout 3.