After a lengthy period of litigation, Bethesda Softworks has fully claimed the rights to the Fallout license in a settlement with Interplay. When Bethesda originally purchased the Fallout rights, it agreed to allow Interplay to keep the rights to create a Fallout MMORPG, with the provision that Interplay secured thirty million dollars in financing and began full-scale development on the game by April, 2009.
As Interplay's general progress on Fallout Online could be summarized by the game's official website, Bethesda sued to void that agreement in 2009. The suit has now been settled in Bethesda's favour. Bethesda will pay two million dollars of "consideration" to Interplay, and Interplay is no longer allowed to develop any games under the Fallout licence. According to the original agreement, Interplay is allowed to sell the early Fallout games until 2013, at which point all Fallout rights are transferred to Bethesda.
At this point, Bethesda has made no announcements about any upcoming Fallout games, with the exception of the Ultimate Edition of Fallout: New Vegas. We don't know whether Bethesda plans to develop its own Fallout MMORPG, but it certainly has plans to continue developing games under the Fallout license.