The announcement was made in Runic Games' forums informing their community first on the release date.
The post also mentions that the news will be properly announced to the public sometime tomorrow.
Excited for Torchlight 2? With the game priced at only $20, I might just give this game a chance.
You can pre-purchase the game on Steam, pre-purchasing Torchlight 2 will grant you a free copy of the first game.