According to papers filed in the court case between Interplay and Bethesda, the Oblivion developer has been working on a "World of Warcraft type MMO."
Development on the game has apparently been ongoing since late 2006, with "tens of millions of dollars" invested and "close to a hundred people" working on it.
Bethesda is allegedly looking to pull the information revealed in the case.
So far, Interplay has in development Project V14, which is rumoured to be a Fallout MMO. Remember that Interplay owns the rights to develop Fallout MMOs; if the Bethesda game is Fallout-based, it won't legally be able to sell it.
Bethesda issued a lawsuit on Interplay last September for breach of agreement and infringing the Fallout trademark.