Found a blurb at MacGamer that points to an interview with Bungie's Matt Soell (he of Matt's Weekly Update fame). Dane Baker asks a good and lengthy set of questions about HALO, such as:

CoreMagazine.com: Are there new elements that Bungie is implementing for the Xbox version that aren't in the PC or other versions of the game? If so, give us an example of a few of those features/elements.

Matt: If you look at the screenshots or footage from the old builds of Halo as opposed to the newer ones, you'll get a sense of some of the things the Xbox allows us to do graphically. PCs will supersede that eventually, of course, but we completely retooled our engine to take advantage of what the Xbox can do.

Check it out, there's tons of cool stuff to read.