Runic just announced via their Torchlight II Facebook page that the game will be releasing as a digital download September 20, 2012. The best part is they included a link to the official launch trailer, which I'll link below.
The game will be available from a variety of sources: Runic Games, Steam, Perfect World, GamersGate, GameFly, and GameStop, so wherever you prefer to get your games, they have you covered. Having had a chance to play this game for myself, I can tell you it's a blast to play, and will be worth picking up even if you haven't experienced the first game. Who doesn't need a little dungeon crawling in their life?
Torchlight II will release for Windows PC September 20, 2012.