In this video, creators Harvey Smith and Raphael Colantino discuss how the game works. It basically sounds like a steampunk Bioshock mixed in with elements of Deus Ex, such as being given missions and then total freedom on how you execute them, varying between stealth, combat and information-gathering. Potentially interesting is an eavesdropping system, where pausing and spying on enemies before engaging them could lead to your objectives changing on the fly.
Originally mooted for summer 2012, a final release date for the game has not yet been revealed. It will be published by Bethesda who, in the wake of the enormous success of Skyrim, are probably going to be content to wait until Dishonored is fully ready before releasing it.