If you were worried that Ubisoft would have an epic brain fart and not bother to follow up on the commercial success of
Assassin's Creed - fear not. It has been confirmed that a sequel is in development.
This was revealed in a Ubisoft conference call to analysts. It was Yves Guillemot, CEO of the publisher, speaking in response to an analyst asking whether we will see the game in Ubisoft's fiscal 2010. "We're not answering that question", Guillemot said. "What we just can say is that we are working hard on the product."
That's pretty much it in terms of detail from Ubisoft. It has been suggested elsewhere by Wedbush Morgan analyst, Michael Pachter, however, that the game will take the action forward a few centuries. Pachter, who apparently gets told all sorts by publishers and devs, says the game will take place in the 17th century and may involve the French Revolution.
Sources:
GameSpot
Giant Bomb