Iron Bars

Iron Jail 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 (Which were revealed in the same Image, posted by Jeb on his Google+ account on July 1st 2011). They are likely to serve a similar purpose to fences, although with a very different appearance.

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