So apparently Arcane Studios, with a little help from Half-Life 2's Art Director, is working on a new kind of shooter. They're trying to fuse singleplayer with multiplayer in the hopes of creating something we've never seen before: crossplayer.
I can't see any explanation as to how this is going to be implemented, but it sounds interesting. One of the many things I'm looking forward to seeing in Spore is the way they use other peoples' creations in your single player games, which is sort of single/multi player.