Before Bethesda developed Fallout 3, a title of the same name was already well on the way. Codenamed Van Buren, the project was helmed by Black Isle Studios, who also developed by the first two games of the series.
With the engine almost completed, Van Buren (Fallout 3) was untimely cancelled in 2003 as Black Isle Studios was closed down due to the financial troubles of its parent company, Interplay. The Fallout intellectual property was sold off to Bethesda, who went on to developed the version of Fallout 3 we know today.
The just announced Fallout: New Vegas is a new entry to the series. The game is being developed by Obsidian Entertainment, which is made up of former members of Black Isle Studios. The staff included Josh Sawyer, the lead designer of the canceled Van Buren (Fallout 3) in addition to being project director and lead designer of Fallout: New Vegas.
To further add fuel to the fire, Van Buren (Fallout 3) takes place in the Midwestern area of the United States. The name implies that Fallout: New Vegas will be set in the same general area.
Whether Fallout: New Vegas is actually Van Buren (Fallout 3) or a completely new title remains to be seen. Expect more information to be revealed the closer the 2010 release date gets.