Yesterday, Atlus sent a note to its fanclub, the so-called "Faithful," promising to maintain Demon's Souls' North American servers until at least March 2011. I suppose that you could play it as a single-player game, but that'd be missing the point of Demon's Souls' persistent not-quite-MMO presentation and feature set.
When those servers finally do shut down, Demon's Souls will automagically get more difficult: most of the games' multiplayer components -- the bloodstained messages, the ability to summon another player into your game -- are designed to mitigate some the game's crushing brutality.
To celebrate the announcement, Atlus has set the Demon's Souls' world tendencies to pure white -- enemies have less health and drop healing items, but they also provide less souls and crappier items.
You can pick up Demon's Souls for a cool $30 at most stores now, so you really might as well -- you'll have at least nine months to play, and your first foray into Boletaria will be slightly less soul-crushing.