This is pretty well established in New York where Foursquare has been live for a while, but there is a lot of potential for smart small businesses to join in. According to the Businesses section of the Foursquare site, so far the list in the UK looks like this:
+ Bea's of Bloomsbury (London, UK)
+ Dose Espresso (London, UK)
+ Hummus Bros - Holborn (Camden Town, Greater London)
+ Hummus Bros. (London, UK)
+ Kaffeine (London, UK)
+ Paul A Young Fine Chocolates - Bank (London, UK)
+ Paul A Young Fine Chocolates - Islington (London, UK)
+ Pumphreys Coffee (Blaydon on Tyne, Tyne and Wear)
But I think it will grow rapidly over the next couple of months