Project V13, but now we get word that Masthead Studios will be involved in its development, according to an article at Eurogamer. Apparently, Interplay will still be the official developer of the game, yet they will be using Masthead's technology, tools and resources. Of course we all know and love Masthead Studios for their upcoming MMO (also post-apocalyptic) entitled Earthrise.
Project V13 has been in development since November of 2007 and was in the news several times for various staff changes such as the inclusion of original Fallout designers Chris Taylor and Jason Anderson. Anderson has since left, but all signs point towards this "top secret" project being some inception of "Fallout Online". After seeing what Masthead has done with their own game at GDC last week, we're now even more excited about what a Fallout MMO could become.