It had its world premiere, to great acclaim, at the Venice Film festival on 5 September 2004 - the first animated movie to compete for the Golden Lion in thirty years. The latest news, including a poster and a movie trailer, can be found at http://www.nausicaa.net/miyazaki/newspro/current_news.shtml . The official web site, which is all in Japanese and doesn't have much content but does have a good copy of the poster, is at http://www.howl-movie.com/ .
The plot of the movie roughly parallels the plot of the book until Howl sends Sophie into Kingsbury to black in his name, at which point it diverges completely. Martha Hatter doesn't exist, although she is alluded to in passing. Michael Fisher is replaced by Markl. Other characters from the book or not present in the movie include Miss Angorian, Wizard Suliman, and Mrs. Pentstemmon, although the last two have some of their traits combined in the movie-only character [Madame Sulliman]?.