According to MuggleNet, Warner Bros. has confirmed that their casting department is just about to make final decisions on key Harry Potter and the Goblet of Fire characters, but are not ready to announce them yet.
Also, a fan of the Harry Potter movies applied to be in the next movie and received a letter in return (which was posted at SnitchSeeker.com) saying that all casting for Harry Potter and the Goblet of Fire had been filled and that casting for Order of the Phoenix is not scheduled to begin till 2005.
Here is what the letter from Fiona Weir, who is the current casting assistant for Goblet of Fire said:
Thank you very much for your letter. At present we have all our requirements filled for Harry Potter and the Goblet of Fire. However, should this situation alter in the future your details are on file to be referred to. Unfortunately we cannot accept applications outside of the United Kingdom.
Casting for Order of the Phoenix will not begin until next year and I can only suggest for you to look out for further notices on the official Harry Potter Website. In the meantime I hope you enjoy the next film - Harry Potter and the Prisoner of Azkaban - when it is released this June.
I would like to thank you again for sending in your application and your interest and enthusiasm.
Wishing you all the best for the future!
It would seem from this letter that Warner Bros. may indeed be doing Order of the Phoenix, but this has yet to be confirmed. We will update if we hear anything.