Iron Bars

Iron Bars are a possible addition to the 1.8 "Adventure" update. They are a new type of block that seems to occur naturally in ruins (revealed in the same image, posted by Jeb on his Google+ account on July 1st 2011). They may serve a similar purpose to fences, or act like a gate or door.

Although they appeared in the screenshot they are not 100% confirmed as of yet. They have not been named or even mentioned by Jeb or any of the other developers.