Be sure to take this one with a grain of salt, but Sid Meier is apparently very interested in making a Civilization MMO at some point in the future -- according to a recent interview. However, Sid basically goes on to say that he doesn't think that sort of game is his next project, as he's interested in doing some more single-player games (Alpha Centauri 2, please?) before tackling something else.
We've got to admit it though, the concept of an MMO where you start your very own civilization and slowly expand it over time sounds intriguing to say the least. Aside from all sorts of design hurdles, there are some very exciting ideas in the concept. The easiest method of doing a massively Civilization game would be to take a few notes from Guild Wars in terms of subscription-style. That way, a player could work on their civilization for as long as they wanted, take a break for however long they felt like and then come back for expansions or just because they felt like it -- all monthly-payment free.