I think the time of a Halo movie has come and passed. The best time would've been between Halo 2 and 3 (closer to the launch of Halo 3) which is the time period where I feel the Halo franchise was strongest. With the absolutely explosive success of Halo 2 and the incredible hype and anticipation of the first "next-gen" Halo, and names like Peter Jackson tied to the project, the movie would've been an almost guaranteed box office success.
Now that District 9 has been made (by the same director chosen for the Halo movie), the human-covenant conflict and relationship in a Halo movie would seem mundane and boring in comparison to the brilliance of District 9. Plus the style of direction in District 9 would've set Halo apart from all the other video game movies and really made it stand out. Now that it has been done it'll just seem like it's piggy backing on the success of District 9 as a sci-fi/action movie.
