An article on the topic: http://kotaku.com/5674721/when-an-mmo-dies

Doesn't get into specifics of where the data goes, but would guess the servers are boxed up and stored somewhere, with the hope their value could be repurposed. But with most of the people involved looking to cut their losses, stuff is probably moved next to the digital equivalent of the Ark of the Covenant.

But then projects like this spring up: http://mxoemu.info/ So maybe as the world is downloaded onto everyone's machines, they can possibly be re-created.