I know! You thought it couldn't be done! That it would never happen! That this game absolutely, positively would not have Master Chief in it at all! Well, that's not entirely true. We've confirmed at Gamescom 2010 that Master Chief will be in Halo: Reach ... just not in the way you're probably expecting.
Bungie has an elaborate "Player Reward System" in the game, where you earn credits from everything you do in the game, even working in Forge. You can spend these hard-earned credits on armor upgrades that become available to you as you level up. These items are opened up by the community at large as well, and Community Lead Brian Jarrard's hypothetical was example was, "When the community collectively kills one billion grunts, a new tier of armory options might become available."
You can choose to save your credits and purchase special items like Armor Effects and Firefight voices. It determines whose voice you hear while you're playing first person in Firefight. These brand new, newly-recorded voices include Buck (yes, Nathan Fillion is back), Sergeant Johnson ("Suck it up, Marine!"), Cortana (that'll be strange) and ... Master Chief. In fact, Master Chief has more lines in this game than he did in any of the other Halo titles. "You're going to hear him talk more than he ever talked in the original Halo games."
So, there you have it. There's some brand-new Master Chief content in Halo: Reach, but you'll have to pay (by playing) to earn it.