Spotted (in two senses) on the corner of Cromwell Road and Queen's Gate (i.e. outside that cultural megosaur, the Natural History Museum). A quick Flickr search reveals that this is not the only one. Since mid-August, similar posters have been appearing all over the place.
Chelsea Bridge Road
Chiltern Street (Marylebone)
Exeter Street (Covent Garden)
St Martin's Lane
Let us know if you can add to this list, and we'll see if some kind of pattern emerges. And please tell us it's not just another viral marketing campaign by a soft drink manufacturer or something.