Team Meat (the dev duo behind Super Meat Boy), has been working on a secret undertaking called The Binding of Isaac. The side project, which has nothing to do with Team Meat's next "feature length" project, finds McMillen working with programmer Florian Himsl (Coil, Triachnid, C word), and a score by Danny Baranowsky (Canabalt, Super Meat Boy, Cave Story 3D).
McMillen plans to reveal release date and platforms when the title is finished in about a month. The only hints we get for now are, "It's a play on my favorite game of all time," and that it's a roguelike. He hopes to share more information next week.