Ken Harward: We released a mini-SDK a few weeks ago, which allows for all types of content-making (maps, models, materials, etc.) for both traditional single-player and arena-mode maps. Once we have merged with the latest version of the Source engine from Valve, we intend to release an SDK that allows for mods to be made of the game. This will have all the functionality of the HL2 SDK. We're hoping to release this later this summer.
It will be out soon and will work with the newest version of the source engine