n. The act of lying rigidly face down on or over an object with arms straight at one's sides.
Other Forms
Planking, the Australian craze of lying face down stiff in often bizarre places for a photo op, claimed another victim when a man was critically injured after being thrown from the roof of a moving car when his planking went bad.
—“'Planking' goes viral despite death, injury,” Reuters, May 19, 2011
Planking is not at all a new thing. 25-year-old Adelaide resident Sam Weckert claims he and a buddy coined the "planking" moniker in the summer of 2008 to describe their practice of laying down on on dance floors as a prank. But as BuzzFeed points out, planking is just the latest incarnation of the ancient internet meme: The Lying Down Game. The Lying Down Game (LDG) is generally believed to have been launched in 2006 by a couple friends from Sussex, England. It produced its own craze in July of 2009 and was covered by media all over the world. The original Lying Down Facebook page still remains, and dismisses planking as an Australian "copycat."
—Adrian Chen, “How A Facebook Page Turned Into a Deadly Internet Craze,” Gawker, May 16, 2011
2009 (earliest)
Australia, the only place where planking would ever be considered a nation-wide fad.
—daqua_99, “The People of Wal-Mart,” GameSpot Forums, September 18, 2009
The original Planking Facebook page claims that it was founded in January 2009, so it's likely my earliest cite from September 2009 is quite late.

It's also worth mentioning that today (May 25) is World Planking Day, so do what you will with that factoid.