I wouldn't necessarily be opposed to a Fallout MMO, but Bethesda definitely shouldn't make it. Bethesda I think has always been known to be a very single player experience creating games i like to call massivly single player RPGs. Games that are set in huge sandbox worlds, with several related and Parallel story lines and quest arcs and nearly limitless replay value. This should be something Bethesda works to perfect and expand and I believe that an MMO would only divide their attention like it did to Blizzard (can I play Starcraft Ghost yet).
If interplay can make it then they should, but not without help. Obsidian has been a longtime partner of theirs and has a long and diverse history of their own. If I had my way I would like to see SCE do it, but i'm not sure how the microminions will feel about that.
Anyway a Fallout MMO done right could be what finally dethrones Mighty WoW.