Ubisoft is apparently planning to support Assassin's Creed 3 with enough DLC to warrant a Season Pass, based on a letter that the publisher reportedly sent to GameStop locations. A photo of the note, snapped by a store employee, is posted at Kotaku and it appears to be legit given its resemblance to other materials that Ubisoft has sent out in support of the game.
The letter confirms that Ubi created a "complementary development team" that has been tasked with building "post-launch episodic content," DLC that fans will be able to buy either a la carte or in one go, via a Season Pass. That Season Pass will apparently be available for pre-order soon, provided of course that this letter actually is the real deal. I find the mention of "episodic content" noteworthy though; it certainly suggests that the DLC plans involve more than just multiplayer maps.
We'll all find out soon enough. Assassin's Creed 3 comes to PlayStation 3, Xbox 360, and PC platforms on October 30, 2012.