Zelda sage Eiji Aonuma confirmed a few months ago that his team at Nintendo were already planning the next Zelda title, but he's now hinted via the Hyrule Historia book, that it has entered development.
"The long sailing time of creating Skyward Sword, a game to celebrate the 25th anniversary of the Zelda series, has just come to an end," said Aonuma. "The voices of those who've played it from around the world reach us. We hear their admiration comments, and their criticism too. All those voices become energy for the next voyage. Well, to tell the truth, we've already set sail in a new voyage."
While Aonuma doesn't make specific mention that the game is in development, it's a rather strong suggestion and this will come as great news for Zelda fans. Even though Skyward Sword has only just been released, it's always good to look towards the future.