Dawn of War 2 is developed by Relic Entertainment and published by THQ Canada. DoW2 integrates Steam as it's main DRM component and Games for Windows Live into it's online component, and as an additional layer of DRM.
DoW2's mod support allows for custom LUA scripting, editing of game data and creation of custom maps and textures. Editing of 2D menu graphics (scaleform), 3D models and source code is not currently possible. High level source code is never likely to become available (aside from crude third party disassembly). As of patch 1.4 custom map creation is officially supported through Relic's WorldBuilder application.