Here's something I didn't know: Those who pre-ordered Dead Island got a code for free DLC, which includes the "Bloodbath Arena" and a weapon called "the Ripper." They just can't use the Arena until it releases for everyone, evidently.
That will be "later this month," says publisher Techland, with rumor having it around the last week right now. You'll get four maps sending endless waves of undead at you, so, sounds like undead underdome riot.
Bloodbath Arena is $10 if you didn't get the code with a preorder. The version for sale will not contain the Ripper.