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

Bugs

 * Using pick block on mossy cobblestone wall will give the player normal cobblestone wall.
 * Mob AI do not recognize cobblestone walls as obstacles. As such, if cobblestone walls block a mobs path, they will simply walk into them.
 * An example of this is if a house has cobblestone walls bordering an entrance door. Villagers will walk against the wall instead of going around, unable to enter at night.

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 at this point, only crafting use for moss stone.