Interesting interview here with Dennis Crowley, co-founder of Foursquare. Follow the links and there are some interesting practical examples of how Foursquare can be used by businesses. This helps to bring the potential of location-based marketing to life.
For example, if someone comes into your shop and shows you on their mobile phone, via a Foursquare app, that they are an ultra loyal customer, then you might choose to reward that loyalty, and so on. You would probably want to because then they will spread the news online and tell all their friends.
Some other posts you'll find interesting: