It's not zombie-themed, but Red Dead Redemption's "Undead Nightmare" pack will add a new free roam multiplayer mode to the game, called "Land Grab."
Rockstar released the details of the mode today, which has players fighting for control over towns in the game's online world. Seven towns will be up for the grabby grab in this new mode. Just find the post that says "Land Grab" on it, and activate a timer; defend that area for a predetermined amount of time to claim the area.
There's more to it, which Rockstar has detailed over on its official blog, so check it out if you're interested. "Undead Nightmare" is out next week for Xbox 360 and PlayStation 3, and will run you $9.99/800 MS Points