The DLC will see the dead roaming the Wild West, getting up to all sorts of cannibalistic hijinks. It adds a new singleplayer campaign with new weapons, animals and random events. For multiplayer fans, there's also eight new zombielicious characters.
It's still unknown when Undead Nightmare will lurch onto Xbox 360 and PlayStation 3, costing 800 Microsoft Points ($10), but the trailer notes that it's "coming very soon." Around a ghoulish festival of the macabre might be an appropriate time to launch.
This looks like it's going to be a lot of fun and is implemented well. I convinced myself to drop $10 for that liars & cheats dlc, and this appears to be a better value for the same amount.