Bethesda's Pete Hines told a room full of gathered media that the company is working on a new Fallout game called Fallout: New Vegas. The game is being developed by Obsidian Entertainment and is planned for release in 2010 on PC, PlayStation 3, and Xbox 360.
There aren't many more details than that, but Hines also mentioned that the game wasn't a sequel to Fallout 3, but it is the same sort of game. Also, he noted that it had no impact on what Todd Howard and the folks at Bethesda Game Studios were working on, which hasn't been announced yet.