Brandalism: UK public art-public space project
See "Brandalism: Street artists hijack billboards for 'subvertising campaign'" from the Independent, the Brandalism website, and "Brandalism,” The World's First Collaborative Act Of Ad Subversion" from Fast Company Magazine.
Space Hijackers, Olympic Village, East London. One of the complaints about the Olympics in London is that people and organizations who don't pay big fees to the Olympics can't use Olympics-related words in advertising and other settings. E.g., see "Who’s ripped off now?: Sponsors get very, very angry about other people being allowed to play with words" from the Financial Times.
Just Loot It, Bill Posters.