Though little is known as of yet, a piece of concept art popped up recently (see above) that more than hints towards the project being Fallout-related. Nuka Cola anyone?
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Rumor: Interplay working on Fallout MMO
08.04.2009
When Interplay first sold the Fallout rights to Bethesda to create Fallout 3, they retained the rights to create an MMO based on the franchise. Recently Interplay brought Masthead Studios to help work with them on the mysterious game, codenamed Project V13.