It's only free if you bought the Hardened or Prestige version of Black Ops. If not, it's included with the four zombies maps that were originally included with those editions, for 1200 MSP.
Wow, who ever thought that Activision would give away Call of Duty content for free?
The four "remastered" maps were previously included with the Hardened and Prestige releases -- if you already have one of those, the rest of this content will be free. Otherwise, it's the standard 1200 MS Points/$15. It'll be easy to acquire, too, according to One of Swords: " the short version is you'll be able to go into your Download History and re-download the classic pack you already own, and the fresh download will include the new Moon map. And you'll also get the soundtrack and theme, too."