Gearboax has released this Borderlands 2 trailer showing off the Creature Slaughter Dome. Borderlands 2 will have a sweet pre-order bonus for gamers who put their money down at GameStop. The trailer asks you to survive Pandora's creatures, so my guess is Creature Slaughter Dome is similar to horde mode with waves of skags and other Borderlands baddies.
I already pre-ordered my copy of Borderlands 2 so this is just icing on the cake!
Borderlands 2 will be released on September 18, 2012.