I've seen this concept knocked about for a few years and I still wonder why you would want a Halo movie. I concede that I've only played Halo 3, but while I loved the game and think its among the best I've ever played, I found the story quite stupid. It's written like a middle school scifi fan would write it, inscrutable and filled with self-important twaddle.
Would I like an Alien-style movie with Master Chief type armored guys blowing stuff up? Sure. But not Halo: The Movie.