This is a strange one, but it appears Microsoft has patented the ability to drop-in and out of co-op games - think Gears of War.
Patent #7,559,834 was invented by a James York back in 2002 but it was assigned to Microsoft on July 14th 2009.
Dynamic join/exit of players during play of console-based video game
A squad-based shooter video game allows players to dynamically join and leave the game, while that game is in progress, without the players having to save and restart the game. When a new player joins an in-progress game, a new squad member is allocated to the new player and the screen is split to present a viewing panel for the new player that depicts scenes from the perspective of the new squad member. When an existing player leaves the game, the screen is unsplit to remove the viewing panel for the exiting player and that player's squad member becomes part of the squad being controlled by the remaining player(s).
However, if you think that now no other company can use this technique, the patent does strictly say that it is only for squad-based shooters.