If you’re wandering along Oxford Street on a Friday feeling in need of a caffeine injection head on over to Debenhams, check in with Foursquare and collect your free coffee.

Debenhams is one of the latest national retailers to get on board with the emerging location-based social network Foursquare. If you’re not familiar with this new phenomenon, the basic idea is that you ‘check in’ to real-world places from your GPS-enabled mobile device and earn points for doing so. And what do points mean? Well, free coffee in Debenhams’ case but the possibilities are endless for retailers to utilise this service to retain their customer base and gain new customers.

When you check in, your foursquare friends are notified of your whereabouts (hmmm… a bit Big Brother) and you can automatically Tweet and update your Facebook status too. There are other features of the service like ‘tips’ (if the place you’re in does the most awesome hangover bacon butties) to help you spread the word about what’s great … and what’s not so great about where you are.

Of course, the potential for bricks & mortar retailers to capitalise on this are significant. Offer discounts, freebies, new services to loyal customers who check-in with foursquare and they’ll spread the word for you. And hopefully a good word at that.

Foursquare in the UK is in its infancy. But it looks set to become big in 2010. Who knows, with national retailers like Debenhams coming on board, it might even see growth like Twitter saw last year. Watch this space.