By way of a new job posting for a game director for an "unannounced project," Blizzard seems to have pretty clearly announced that this project will be a new Diablo game (thanks Ars Technica). The responsibilities mention a knowledge and passion for Diablo games and experience "developing online games, action games, and / or role-playing games," qualities some feel may not have been present enough during the initial design phases of Diablo III. Aside from those details, the abstract makes it sound pretty apparent that a new Diablo game is in the works:

Blizzard Entertainment is seeking a game director with outstanding communications skills, proven experience in creative direction, strong ability in system design, and a stellar track record of shipping AAA games to lead the Diablo series into the future.