Keith says it was sent yesterday (Feb 14th), which is after the creation date for the "2012 Climate Strategy" memo of "2/13/2012 12:41:52 PM." So, indeed, it's possible the document was first scanned at (or by) the Heartland Institute on 2/13, as per its document properties, and then emailed to the leaker either Monday afternoon or Tuesday morning, who then quickly sent it around.
So no clues there. Except, if this is the scenario, I'm surprised at how little time Desmogblog or Thinkprogress Green spent in analyzing and discussing the documents.
By the way, Thinkprogress is not hosting the IRS document. Legal concerns, maybe?