From the Q&A session:
...Colfer said he likes the darkness and drama of the latest Artemis Fowl books but the next book will get a lighter, Woody Allen-type character. He noted that only boys think Capt. Holly Short is too cocky. He also said the title of the fifth book would be Artemis Fowl and the Sorcerer's Goblet of Azkaban, then he backtracked and said it would be called The Last Colony. It's due in late 2006 or 2007.
He is a very diverse writer. The article reported he is also working on a crime thriller and a series for young children.