Back in October of last year it was revealed that Peter Jackson signed on as executive producer for Halo: The Movie, and the project was tentatively scheduled for summer 2007. That date now seems to be premature as IMDb has updated its movie listing reflecting Master Chief's silver screen debut has been postponed until 2008. While a simultaneous launch of both the movie and Halo 3 would be beneficial for both parties, Bungie always claimed the two are not connected in terms of release date, so this does not automatically mean that Halo 3 is delayed as well.
News source: Worth Playing