This essay by Daniel Blackburn of Carbon Based Games looks at just that--how Bluetooth can enable social gaming by making players live in the game, depending on chance interactions with other players for advancement. GPS games like mogi allow this to some extent, but the possibilities with Bluetooth are rich.
Blackburn suggests game concepts from tamagotchi style games to trading cards; we can take this a few steps further and imagine Animal Crossing: Bluetooth or even an MMOG that causes you to physically steer clear of other players if you want to avoid combat. This sort of game-reality mix is also perfect for ARGs with the budget and resources to take advantage of it.
[via pasta and vinegar; image of mogi]