Since the web so far contains no references to both the term MacGuffin and the term Social Object I thought I'd make the obvious connection.

A MacGuffin is Hitchcock's term for an essentially meaningless story device that's there simply so that the story has something to be about. Social Objects - as I rationalize the theory of them - is the same thing for phatic expression*: Something for us to be people about.

Cf. also that wonderful scene from The Last Tycoon where Monroe Stahr acts out a scene in his office - the scene revolves around a penny placed on Stahrs desk - but other than capturing the interest of the onlooker and occasionally the characters it has no meaning. The guy Stahr performs the demonstration for eventually asks "What's the penny for?" - "It's for the movies".

*(thanks to Lisa Reichelt's Reboot talk for fixing this term in my mind for that kind of activity