The line between viewer participation in contemporary art has melded completely in the Seville Biennial. For its Biennial, the city of Seville has created a link between the internet and real-world by allowing people all over the world to change the colors on a tower. The project, "Colour By Numbers" is a light installation in the citys Tower of Perdigones.
Like the earliest Web-TV show I recall (YORB), you affect change elsewhere via the keypad on your phone. Calling the listed number, you control the color of the lights on each floor of the tower through a code on the website. You can watch the result via a live web feed.
Check it out: http://colourbynumbers.org/