"What you're seeing is a piece of flat, platy, layered sulfur-rich outcrop rock like we've seen almost everywhere the Opportunity rover has been in Meridiani Planum," said Bell. "Sometimes, like in this case, those flat, platy rocks have been tilted or dislodged, this one probably from the forces associated with the huge impact crater that formed nearby."
I'm with NASA on this one. The "wood plank" has always struck me as one of the lamer Martian "anomalies"; weirdly, its defenders are disproportionately shrill. None of which is to say the "plank" doesn't look like wood. But when seen in context, the sense of mystery vanishes -- which is more than I can say for other, more insistent surface mysteries.