my brother is possibly the greatest leading indicator of social trends of anyone i know. he has an incredible knack for picking up and getting very involved in cultural trends which ultimately become exceedingly popular in the mainstream years later. he is the ultimate early adopter. so, when danny started getting very very involved in reenactments and roll playing games a few years ago (larping, elarping, etc) i stated to pay attention, even if i didn't personally get the appeal.
i am not sure we are 100% there yet, but listening to dennis and naveen discuss foursquare last monday at the new york tech meetup, and then starting to play the game myself - i think that, yet again, danny called the trend early... sure, the local 'game' itself of 'foursquare' is interesting itself -- but what is far more interesting is that it seems that foursquare is meant to be extended in many different directions.
it feels like foursquare is the platform for mainstream elarping, where individuals, organizations, and maybe even companies, can easily create their own games to overlay on top of reality.
of course, there are plenty of other people who have called this trend before danny. for instance, a future full of args is a major theme of rainbow's end; however, i give my brother credit yet again for adopting a few years before the tech is real, and the world is ready to play.
of course, i also give dennis and naveen incredible credit - not for foursquare as the game that currently exists, but for creating what appears to be the beginnings of a platform/ecosystem for the mass adoption of an old, but possibly soon to be mass form of virtual/physical gaming...