Last i have read its on hold because Microsoft and someone couldnt agree on the storyline. Its still on hold but i read that Steven Spielberg was planned to direct the movie now, not sure if that was finalized or still up in the air.
Heres what i found on the internet about the problems with the halo movie:
The script is written, but disagreements over distribution rights have repeatedly sent this project in and out of pre-production. Some of those involved have said the project is "dead", but Peter Jackson, the executive producer, continues to claim that this film will get made. In other words it's on hold.
http://www.imdb.com/title/tt0464037/faq