The Nuketown Zombies bonus map for Black Ops 2 is open for business on the PS3 and PC today, available as a free download to all Season Pass holders on each platform. Xbox 360 Season Pass holders have had access to the Nuketown Zombies map since mid-December.
The Black Ops 2 Season Pass costs 4,000 MS Points ($50) and includes four DLC map packs on top of the aforementioned Nuketown Zombies map, plus an in-game calling card. None of the additional map packs have been released yet, though they're all due in 2013.