We're still waiting for an official announcement concerning Assassin's Creed IV: Black Flag, expected to be delivered by Ubisoft later today. First word on the sequel to Assassin's Creed 3 came in the form of a promotional poster, depicting a 'pirate Assassin' aboard an 18th-century ship.
(http://img.gawkerassets.com/img/18fv4w0yrkdlyjpg/medium.jpg)
The world will apparantly feature Cuba, Southern Florida, Jamaica and other Caribbean Islands, with the date of the game's events implied to be either during or beginning in 1715. That year falls within the era known as the 'Golden Age of Piracy,' so the earlier image of a sword and pistol-wielding Assassin fits quite well.
(http://cdn.gamerant.com/wp-content/uploads/Assassins-Creed-4-Black-Flag-Caribbean-Map-570x366.jpg)
Exactly what story the developers have cooked up will have to wait until the official announcement.
Assassin's Creed IV: Black Flag is expected to release in 2013 for all major platforms.