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.

On July 11th 2011, Notch revealed 2 new Screenshots of the Strongholds (In 2 different settings of Brightness) where the Bars, or more likely, Chains, Vines,or Dry Sugar Cane can be clearly seen, aswell as the strongholds, the 3 other new blocks and a dead skeleton next to the zombie(it has been zoomed in numerous times). Also, the lighting changes have been explained a bit(may not be 100% correct) by a user on the forums