When it's no longer a Bethesda game. (Also, a completely different engine, built from the ground up for multiplayer.)
...also, get your dirty multiplayer off my singleplayer games. There's few enough SP games as it is, what with MP being crammed into so much stuff where it's not needed/wanted.
You can discuss multiplayer ideas in Fallout here: http://www.gamesas.com/topic/1512524-fallout-multiplayer-discussion-thread/