Assassin's Creed 3 will be set during the American Revolution, according to CVG. According to a source "close to the project", Ubisoft will announce the first details on the game, including the setting, by end of the company's financial year, which ends March 31st.
Ubisoft has already confirmed that a new "major" Assassin's Creed is planned for release this year, all but confirming that "AC 2012" will be Assassin's Creed 3. Ubisoft has also confirmed that the new game will feature a new setting and a new protagonist, speaking to rumors that the series would drop the historical setting to focus solely on the series' overarching hero, Desmond.
The American Revolution has been a chronically rumored locale for the series. Assassin's Creed Brotherhood's "Da Vinci Disappearance" DLC featured the co-ordinates of New York City, increasing rumors that the next setting would be, at the very least, set in the US. More recently, a survey from Ubisoft asking fans what scenarios they'd like to see in AC3 included "the confrontation between British colonists and native Americans during The American Revolution."
Unlike most rumors, this one has an explicit expiration date, so we'll get the skinny (or not) some time next month.