There is no official release date. There is no estimated official release date. There is no unofficial or rumored release date. Some magazines and software retailers will provide a release date for Halo for various reasons, but the official word is that any date that didn't come directly from Bungie should be assumed to be wrong. Halo's release date is planned to coincide with that of the X-box, as Halo is a major launch title for that console.
Nothing is known about a demo version of Halo.
If there will be one, it will probably be released close to the full product's release date. Halo will be available on the X-box, Macintosh, and Windows. Contrary to earlier rumors, Halo will not be ported to the Playstation 2. Halo will ship on DVD for the X-box, and most likely on CD for computers. Bungie apparently doesn't feel that the installed base of DVD drives is large enough to support a DVD-only game.