Fallout New Vegas was recently announced.  It will be released for the PC, PS3 and Xbox 360 next year.  It's not going to be a Fallout 4 though.  It's supposed to be a spinoff game in the series.

I think its going to be like a giant standalone expansion.  Just like how Rockstar games created GTA3, then Vice City and San Andreas before finally releasing GTA 4.  Either way, I think this game is going to be sweet.  Fallout 3  is definitly one of my favorite games, and if they follow the same formula of an immersive environment this has to be a great game!