The Bethesda folks weren't kidding when they announced plans for a Fallout 3 mod kit for the PC the other day—the software is already out. You can grab the Garden of Eden Creation Kit over at FileShack, and surprisingly, the download is only around 8.25MB.
According to Shacknews, the GECK will let you "create, modify, and edit any data for use with Fallout 3, from building landscapes, towns, and locations to writing dialogue, creating characters, weapons, creatures, and more." You can find documentation in the official wiki, which contains a tutorial showing how to create your very own vault. Apparently, you can use the game's prefab structures to create authentic-looking layouts in a relatively short amount of time.
Shacknews has also gleaned a few details (and posted a few screenshots) of the first upcoming downloadable episodes, in which players will relive the fictional liberation of Anchorage, Alaska from the "Red Chinese" through in-game virtual reality simulators. This episode will cost the equivalent of $10 on the Xbox 360, and it will reportedly include four to five hours of gameplay focusing "mainly on a combat/stealth path."