I just played Halo 3 yesterday, and I think they will do it, but it will really take a long time. Imagine it, Bungie has discontinued their Project with Microsoft, meaning no one will support them, having Microsoft to sponsor such is a great deal.
I'd give it til 2011. Halo rolls in more dough than most Microsoft games, so not only would making a movie be great for the fans, they will make LOADS of $$$$$$