In a surprising and interesting move, Bethesda (developers of Fallout 3 and current owners of the Fallout license) have announced that Obsidian Entertainment will be developing Fallout: New Vegas. Bethesda will still be publishing the game.
Some of the Obsidian team members were on the team of developers at Black Isle, who were on responsible for Fallout and Fallout 2. Obsidian is well known as the developer of Neverwinter Nights 2, and Knights of the Old Republic II. Bethesda made it a point to note that they approached Obsidian about this opportunity, not vice versa.
Few details have been released so far. It has not been announced which engine will be used, or even if the game will be first-person. What we do know is that it will be an RPG (as opposed to a tactics game), and the plot will not be part of the official Fallout canon.