Ubisoft has announced Assassin's Creed 3 and they even announced its release date. You'll be reunited with Desmond on October 30, 2012. The confirmation of Assassin's Creed 3 comes from an Ubisoft investor call. The entire AssCreed series has been building up to 2012 and the doomsday clock, so needless to say, I'm stoked.
We don't know the setting of Assassin's Creed 3 but we'll be sure to let you know when we do.