GamesRadar recently did a side-by-side comparison of the locations found in the Fallout 3 version of Washington D.C. with the real world locations. Just how well does the game version compare to the actual thing.
A perfect resemblance! Okay, so the Washington Monument is basically just a stick of marble, granite, and sandstone (thanks Wikipedia!), and if they couldn't get that right they should have quit making games to work on basic motor control, but it's a good start.
The site compares the most notable landmarks found in the game. For the most part, the buildings are quite spot on!