To quote Interplay’s Eric Caen:
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“We sold the Fallout IP to Bethesda in exchange for a certain amount of cash and the right to do the Fallout MMO,” he said. “If they refuse to let us do the game, then the sale of the IP is terminated, and they will be allowed to do only one more Fallout, 5.
“The original licensing deal was for three games and their DLC. So they already did Fallout 3, then Fallout: New Vegas, and they can only do one more Fallout, 5, if the sale of the IP is cancelled by the court.”
“When we sold the IP, it was clear that the MMO could be developed by Interplay. Then we received a unilateral termination letter. Since that, we are only asking either to continue the development, either to get this contract cancelled as the consideration would not have been received in full.”
The MMO is aiming to be released in 2012, with a beta earlier that year. However all of this is still in legal limbo, so take what has been said with a grain of salt, as the court has yet to make a decision either way.