I think that although the movie would make quite a lot of money if it was released. it would still be a failure for the games fans. It's a bit like The Simpsons movie really; everybody is going to go and see it - but, let's be truthful - it's gonna' suck.
The gaming genre of Halo cannot in my opinion be transferred onto a cinema screen, part of the magic of Halo is being involved in the game. Without the interactive aspect Halo would be nothing.