It's actually not an MMO and Chris Roberts has mentioned it in numerous interviews. It will have a persistent world, but it'll be a single player experience.
From their FAQ:
"No! Star Citizen will take the best of all possible worlds, ranging from a permanent, persistent world similar to those found in MMOs to an offline, single player campaign like those found in the Wing Commander series. The game will include the option for private servers, like Freelancer, andwill offer plenty of opportunities for players who are interested in modding the content. Unlike many games, none of these aspects is an afterthought: they all combine to form the core of the Star Citizen experience."
/mukkel