Definition: (wikipedia definition) Planking is done by placing the arms down by one's side and laying rigid somewhere and then taking a picture for others to enjoy.
So now even the newspapers don't know how to use the verbs "lie" and "lay"?theage.com.au said:Planking involves participants laying flat on their stomach
By the way, the Wikipedia article actually says: "Planking is the action of lying face down with arms to the sides, in unusual public spaces and photographing it." At least the person who wrote that understands the difference.