The film sequel itself was announced maybe a year or so ago, but just recently they have confirmed the returning voice cast plus a few new actors.
Terry did have an idea for a Maurice sequel (The Redoubtable Maurice) featuring Maurice as a ship's cat - I think the filmmakers have taken that idea and done their own thing (using their own title: The Amazing Maurice - The Waters of Life).
Producer Andrew Baker is a big fan, which is why the first film is reasonably accurate to the book, we have an interview lined up with him and another producer for the Australian Discworld Convention in April, so hopefully he'll be able to share a bit more information about this new film then!