The game, Record Run, uses players' own music libraries, with the game's protagonist running in time to whatever songs you choose. Your songs may influence the game in other ways, though it's not yet clear how.
It's also unclear when the game will be released, or on what platforms.
Record Run isn't Harmonix's first mobile game, but it is its first free-to-play game for smartphones and tablets. (via Gamasutra)