I don't know if this has ever been done before, but a local geocacher in Charlotte has just put out a new interactive cache, Shallow Esophagus. This cache requires you to visit a pay phone, send a text message and await further instructions. I have never personally seen a cache like it and would propose it make a "highlight reel" on the geocaching blog, but perhaps it's old news in other areas. If you're ever in the Charlotte area (big race this weekend!), this should definitely be on your short list!