Ubisoft has made no secret that Assassin's Creed will be an annualized franchise. This year's iteration of the game is supposed to mark the end of Desmond's story, due to a 2012 apocalypse predicted within the franchise lore.
"In Assassin's Creed we set up a timeline with this whole end of the world plot of December 2012," Ubisoft's Alexandre Amacio explained. "That's fast approaching, and the story we have to tell, we obviously need to do it before we arrive at that point."
However, all will be revealed in the not-too-distant future. According to Kotaku, Ubisoft has announced an October 30th release date for this year's release. "Assassin's Creed 3 is the true next generation of both the Assassin's Creed brand and interactive entertainment/storytelling in general," Joystiq caught Yves Guillemot saying.
A first look at how Desmond's narrative will wrap up is likely to come in Assassin's Creed Revelations DLC, leaked this morning.