The people at Machinima, a site devoted to those who make movies with computer gaming technology, expressed their enthusiasm for the idea of making movies in the Halo engine. Their coverage starts here and they specifically address Halo here. Machinima reports the existence of a demo editor which will ship with the public version of Halo. Take a look at this blurb:

Well, let's be honest here. To me at least, the highlight of the show from a Machinima creator's perspective was a total surprise. Sure, I've heard of Halo (via Tritin Films' re-worked version of the game's trailer, if nothing else), Bungie Software's upcoming, graphically astonishing FPS (and have been hoping that it would be possible to use it to create movies somehow), but the announcement of the show as far as I was concerned was that Bungie would be shipping the game with a fully-featured demo editor . An editor they used to create the gameplay movie shown at E3 itself, no less.

A potentially important revelation for aspiring filmmakers in our community if it comes to pass. Kudos to Hugh Hancock for picking up on this, and to HBO for reporting it first.