So AC3 art has started to pop up on the internet, with
a rumored official reveal on March 5th more information coming out on Monday according to Destructoid who said Ubisoft officially confirmed this, although I'm too lazy to find where Ubisoft did so.
First we have some promotional art.
And then we have the official box art from Ubisoft.
Looks like AC3 will be happening during the war of independence, with the new assassin potentially being part native american (that or he's just partial to using a tomahawk and bow).
Also apparently the game has been in development for three years, so we might be seeing the same kind of overhaul that happened between AC1 and AC2 (which will probably be the only thing that will convince me to buy the game when it releases in October).
edit- Also apparently Game Informer briefly had this
this banner image.