For the wiki definition, have a look here.
I came across this fad last night while perusing one of the many message boards I regularly frequent. Like many, my first thought was, "That's about the dumbest thing I think I've ever seen." It just seemed like something people do with too much time and not enough reading material.
But then I began looking around for more examples of this game to get a sense of its scope. What I found was, quite frankly, some of the most brilliant and hilarious photos I think I have ever seen. Here's a sample.
The Taj Mahal Plank
The Great Wall of China Plank
The Rosario on Jimmy Kimmel Plank
The Statue of Liberty Plank
The Overhead Compartment Plank
As of now, this is as close as humanity has come to participating in a worldwide scavenger hunt. It won't be long before we see a plank at The Vatican, planking on the Serengeti, or the Tiananmen Square Plank. It inspires us to get out and see our world with a new sense of wonder and adventure, then interact with that world with a unique sense of humor and aesthetic. Because you have to admit, it's oddly artistic to see someone lying face down in public.
Yup, planking pictures coming soon.