As per this BBC article -
http://www.bbc.co.uk/news/technology-18259306
Seems like a fairly good deal on a lot of things...Nandos/Tesco/Pizza Express/House of Fraser - spend £10 or more and get a £10 credit back
To synch with your card, go here:
http://sync.americanexpress.co.uk/Foursquare
Shame AMEX UK don't promote these things to customers, instead I get a lot of useless rubbish through the post.