At one point, Halo looked on the fast-track for a big-budget Hollywood adaptation. Neill Blomkamp (District 9, Elysium) was attached to direct, but the project was ultimately scuttled due to profit disputes. Blomkamp is still interested in the world, though, and says he would do if it he could have creative control.
"I still really love the world and the universe and the mythology of Halo," Blomkamp said at an Elysium press event reported by IGN. "If I was given control, I would really like to do that film. But that's the problem. When something pre-exists, there's this idea of my own interpretation versus 150 other people involved with the film's interpretation of the same intellectual property. Then the entire film-going audience has their interpretation. You can really live up to or fail in their eyes. That part isn't appealing to me, but the original pieces are appealing."
That seems to make a proper Halo movie unlikely, as Microsoft is protective of its franchise. But 343 Industries has touched on its own less ambitious version with live-action Forward Unto Dawn mini-series, which led into Halo 4.