If the Halo movie had have been made, the final result of the film District 9 would have been completely different, since Neill Blomkamp was initially signed up to direct Halo. Blomkamp left the project meaning a new director was need for Halo the movie to become a reality.
It would be interesting to know who exactly would be interested in helming the project, as it has been suggested that the budget for a Halo film has since been cut right back. Peter Jackson is still dead set on making the project work but with limited funds, it could prove difficult.
Nobody wants to see a Halo film being released that does not do the game justice but Microsoft have to stick to the numbers provided. The screenplay is there and it is definitely a possibility but Halo the movie still seems to be some way from happening.