Foursquare and its other cousin location-based services (LBS) such as Gowalla are getting a lot of media attention. They were a prominent topic of discussion at SXSW (the South By Southwest Interactive and Music Conference held in Austin recently).
Here is an article that gives some good how-to for using Foursquare to grow business as a local company.
http://www.flowtown.com/blog/how-businesses-are-utilizing-foursquare