Wall

Cobblestone Wall is a block introduced in snapshot 12w34 designed for decorative purposes and an alternative to fences. There are 2 types: Cobblestone Walls and Mossy Cobblestone Walls. They work very similarly to fences, as they are 1.5 blocks tall to mobs.

History
Dinnerbone tweets the first image of cobblestone walls.

Jeb tweets another image of cobblestone walls, this time clearer to see

Trivia

 * Interestingly, Mossy Cobblestone Walls name is based on the old name for Moss Stone and not the new one.
 * When a Fence Gate is placed between 2 cobblestone walls (either type) it will stoop down slightly to match the height of the cobblestone wall.
 * This is the first and only so far item that has moss stone as a crafting ingredient.

Bugs
When using the pick-block-button on a mossy cobblestone wall in creative, you'll get the cobble stone wall.