the adventures of a lost phone
As far as marketing stunts go, this one is fairly simple and seemingly inexpensive to pull off, yet completely awesome... which makes it especially wonderful.
BBDO Shanghai, for Time Out Shanghai, placed cell phones in random places throughout the city for people to "find." Once found, they were communicated to via text and told to get in a taxi waiting for them. Those that didn't think, oooh, maybe I'm being kidnapped, jumped in the taxi and were treated to a text message guided tour of unknown places in Shanghai.