You probably already know that "Halo 3: ODST" is a side-story, where you'll play as ODSTs, the soldiers that supported Master Chief in his adventure to save the planet.
Even though it's a side-story, I was still kind of expecting that ol' MC might show up in this adventure. I mean, Gordon Freeman was in all of those "Half-Life" spin-offs, right? Unfortunately Bungie's Brian Jarrard was quick to shut down any chance that he'll be making an appearance.
"Master Chief, at the time this game takes place, is actually off on other parts of the galaxy. So the events that span 'Halo 2' and '3', he's already off doing bigger, more important things. In this game, it's all about you playing the role of an ODST. The way the timelines work in this game, Master Chief is off doing something else."
I guess we'll have to wait until Halo: Reach, which may give us a glimpse at a younger, more marketable Chief.