PvP zone

The PvP zone, a new feature of World of Warcraft: Wrath of the Lich King, is a non-instanced PvP zone with Siege Weapons, destructible buildings, and multiple objectives with lasting effects. Players on non-PvP realms will automatically be flagged for PvP when entering this zone.

The first zone of this type will be Lake Wintergrasp.

From :
 * The Lake Wintergrasp zone will be entirely PvP even on PvE realms, it will not be instanced and it will have sieging.