Treading the waters in the space between between performance art, improv comedy, practical jokes, flashmobs, and public nuisance is a group called "ImprovEverywhere". They're based out of New York City, and they execute "missions" on a mostly unsuspecting public.
These missions have included staged protests (authors picketing the library for being the "Napster of books") to subverting retail chains (dozens of people in royal blue polo shirts standing around in a Best Buy, being generally helpful) to participatory theatre pieces (download this MP3, copy it to your iPod, and don't listen to it until Sunday, when we'll all meet in Central Park and follow the directions in the MP3) to a mock U2 performance (FAKE BONO ROCKS!).
Man, I wish I had 8 more days in the week so I could do fun stuff like that, too.