Yes, there's a sequel to Disney Epic Mickey. A tweet from Geoff Keighley first confirmed the title (of a game his show will reveal on TV as a "world premiere") as Disney Epic Mickey (2): The Power of Two. Then a piece from the AP went ahead and confirmed everything else.
It's coming out for Wii, PS3, and Xbox 360, and features drop-in, drop-out co-op play with Mickey Mouse and Oswald the Lucky Rabbit. Oswald is AI-controlled when another person is not around, and can be controlled at any time by a second player, who then wields Oswald's "remote" ability to control electricity.
Other new developments for this now-official game include full voice acting, including the first voice work for Oswald in Disney history. That voice acting will be used to make the game a for-real musical. "I'm such a geek about musicals," Junction Point studio head Warren Spector told the AP. "I love the co-op and next-gen stuff, but for me, when a character breaks into song, which they do on a regular basis in this game, it's magic."