Legal trouble jeopardizing the rights of developing a Fallout MMO might be in the future for Interplay Entertainment, according to their 2008 10-K report. In a passage under the legal proceedings section of the report, it asserts that Interplay has recently received notice that Bethesda intends on terminating the trademark license agreement between the two companies, due to a breach of contract.
Initially acquiring the Fallout franchise back in 2004, Bethesda agreed to Interplay retaining the rights associated with developing a Fallout MMO, which they're supposedly now doing. As a part of that agreement, Interplay was bound to conditions stating that they were to have both secured funding for the development of the game as well as to have initiated full-scale work by April 4th of this year - neither of which has been met, according to Bethesda.
Interplay naturally opposes the claims, and while there is currently no formal action pending, should Bethesda move to abandon the trademark license agreement, Interplay would effectively lose the rights to their work-in-progress.