PS3Trophies.org, it appears that more Borderlands DLC is imminent. The five recently added Trophies point to three coliseums, called The Angelic Ruins, The Gully and Hell-Burbia, all of which appear to have "lesser" and "larger" challenges.
Eurogamer mentions that Borderlands Creative Director Mikey Neuman was all set to reveal something on Twitter, but it appears the marketing powers changed the announcement to a "later occasion." Publisher Take-Two seems happy with the success of Borderlands and critical reception to the game's first DLC pack, The Zombie Island of Doctor Ned. Reaching out to 2K Games about the DLC, we were informed, "No official announcements have been made regarding future Borderlands DLC at this time."