Dishonored's first DLC pack, "Dunwall City Trials," is available right now on Steam and Xbox 360. It will be available later today on the PlayStation Network following the weekly update. Dunwall City Trials is priced at $5 (400 MSP) and includes "10 challenge maps" that test your supernatural assassination skills.
More Dishonored DLC is expected in 2013 in the form of two story-driven add-ons. What we do know about the upcoming expansions is that supernatural assassin Daud is the second pack's focus. Bethesda has determined with the the assistance of The Outsider that the DLC will be available in "spring 2013."