The story isn't new. When Bethesda acquired Fallout, they left Interplay with the rights to produce a Fallout MMO, as long as full scale development started by April 4, 2009.
According to VG247, it seems that by April Interplay had still to secure the necessary funding for the game, and Bethesda now filed a lawsuit against Interplay for trademark infringement, and breach of agreement over the Fallout MMO. Interplay has disputed the claims and maintains that it still holds the rights to develop Project V13 (the Fallout MMO codename) with Masthead Studios.
Fallout MMO: Bethesda files lawsuit