like someone mentioned above, having a single storage space for the account information would be able to work pretty well (over network, i.e: somewhere on the internet). But then at that point, why not go a step further and allow a whole bunch of servers to link up together, which would allow for even larger users on a single 'server network'.
Kind of like irc with nickserv services.
Certainly at that point servers will then have to take care of:
1) information going to and from users and server
2) information going to and from server and server network hub
And that would require a main hub that manages the flow of information, along with account information.
Leaf servers would most likely require to have a smaller player limit, but by combining so many together, the total amount of players really available becomes so huge. Imagine playing with 1000 users logged on. Imagine the clan wars
Unfortunately that would cause huge docking queues, and probably a lot of computers wouldn't even able to properly display a couple of hundred ships around manhattan... but hey it could still work