As we alluded to in our review of the excellent new Borderlands 2 DLC "Captain Scarlett and her Pirate's Booty," the expansion features one of the most awesome cameos yet in the series.
During one particular battle with pirate-bandits, a large enemy with a drill for a hand and the name "Mr. Bubbles" (with a "Lil' Sis" in tow) appears to fuck up your shit in the most violent way possible. When we returned to the quest later, Mr. Bubbles wouldn't appear, so it seems the Big Daddy from Bioshock (another 2K franchise) only shows up the first time you go through the area.