Iron Bars

Not to be confused with Iron Ingots

Iron Bars are an addition to the 1.8 "Adventure" update. They are a new type of block that seems to occur naturally in strongholds (revealed in the same image, posted by Jeb on his Google+ account on July 1st 2011) and in NPC Villages. They serve a similar purpose to fences.

On July 11th 2011, Notch revealed two new screenshots of a stronghold (in two different brightness settings) where the bars can be clearly seen alongside other common stronghold blocks. The lighting differences have been explained a bit (though not necessarily completely correct) by a user on the forums.

The iron bars can be placed in much the same way as fences or a Glass Pane.

Trivia
a 2x2 area made of the Iron bars can make a hole big enough for the player to fit through, but not rain, water, etc. this can be used as an easy entrance for an underwater house, but not an exit. Glass Panes work similarly.

This is the same sprite as used in Prelude of the Chambered, another game made by Notch.