Jon Heyman tweets that the PTBNL from the Ike Davis trade is expected to be a 2013 draftee, and will be the "key piece" in the deal.
This is pretty much what I guessed yesterday. There was no reason to think the Mets would deal Davis for Zack Thornton straight-up, since they passed on Thornton in the Rule 5 Draft. And it makes sense that the PBTNL would be a 2013 draftee, since those players aren't eligible to be traded until the summer.
The Pirates certainly won't include Austin Meadows or Reese McGuire in this deal, so if Heyman is right, the PTBNL could be someone like second-rounder Blake Taylor, third-rounder JaCoby Jones, fourth-rounder Cody Dickson, or seventh-rounder Buddy Borden. Neither team will speak up about who it is, so the more highly-regarded players from last year's draft class will probably have to spend the next month or two wondering if they're about to be traded.