The Walking Dead: Survival Instinct is getting downloadable content in the form of a "Herd Mode" game type and "Amazon Walker Execution Pack." They'll be available to those who pre-order the game from GameStop and Amazon respectively, though one suspects it'll be available to purchase separately at launch or soon thereafter.
Herd Mode is, as you may expect, a wave-based survival mode, though Activision's not revealed if it's online, or even cooperative in any way. The Walker Execution Pack is a melee weapon package containing a survival knife, sawtoothed machete, kukri, reinforced hatchet, and advanced machete.
This news follows the outing of Aliens: Colonial Marines' Bug Hunt Mode. What is it with games holding back their horde modes for pre-orders?