It would be hard to believe the Rayman Legends video leaked this morning was anything but real, but Ubisoft removed any lingering doubt today. The company issued a statement (excerpted by VG247) confirming the existence of the game. Of course, it's not final.
The leaked video was "intended as a purely internal demonstrative video, and in NO way represents the final game, the final console or their features," Ubisoft said, referring to the game as "Rayman Origins' sequel." The video was meant for "internal production teams who often create game prototypes with work in progress development kits." In other words, it was a demonstration for the people working on the game, not a demonstration of the game.
The upside of this leak is that we can now issue feedback to the team working on the actual game, in the form of this open statement: Hey, that footage looked great. Make the game look like that.
Update: the video has been removed at Ubisoft's request.