Just as a point regarding Demon's Souls leveraging user data, Fable 2 did the same thing in a much more primitive form with the player orbs showing up inside your game. You could talk to eachother within proximity and watch the orbs move throughout your world, and trade items, or invite another orb into your game. It didn't have accurate ghosts, invasions or notes, but in essence Fable 2 was very much a simplified version of the same thing. It might be possible to do the same things, especially now that Live seems to be allowing games to take advantage of higher data rates now that the legacy system is out.