It has been available exclusively on the Xbox 360 since August 23rd, but PC and PlayStation 3 gamers will see the release of the Rezurrection content pack on September 22nd:

"Call of Duty® gamers achieve escape velocity on September 22nd with the release of Call of Duty®: Black Ops Rezurrection, the fourth downloadable content pack for the record-setting title. Coming to PlayStation®3 and Windows PC, Rezurrection celebrates the popular Call of Duty® Zombie saga with everything undead, including the first Call of Duty throw-down to take place on the moon, as well as four remastered levels from Zombies past, an enhanced Zombies soundtrack and new Moon theme.

...This all-Zombies content pack hurls players into orbit with “Moon,” the climax to the Zombies saga, as well as the most challenging and thrilling adventure yet. The undead shift in and out of sight, waiting for the perfect opportunity to strike. Undead astronauts easily shamble toward their next meal, unaffected by the base’s fluctuating gravity.

Arm yourself with a space-age arsenal of epic proportions, including the Wave Gun, capable of cooking zombies from the inside out, as well as the Quantum Entanglement Device, which taps into the theories of quantum mechanics to bless (or curse!) your battle against the undead. Along with new equipment and an all-new Perk for thrice the zombie-slaying fun, you’ll have the tools you need to turn back the undead tide once and for all."

Source: Evil Avatar