Planking - Have you tried it yet?

I'm normally pretty good with internet fads, being right there at the front when they start, but I''ll admit this one passed me by a bit so if you've already heard of it my apologies for todays post.
For those of you that haven't 'planking' a variation on an old game called, imaginatively, the lying down game, is the latest craze to be sweeping the internet.
But just what is it?