As a fan of the old series, I can't help but be overwhelmed with curiosity. The series definitely has the infrastructure to support an MMO, it's just upto the player and the community to provide the RPG immersion. I doubt there would be 500k+ congregated at any one location at any one time, but maybe I'm just naive. If there was an emphasis on a great expansive wasteland then the population could easily be more scattered, I honestly don't know. But more than anything I am curious at this point. In my opinion it has potential, at the very least.
Using instancing, population can easily be controlled in each zone. There would need to be an mechanic to switch instances so you could be with firends, but for something loke Fallout, a menu command would do fine.