From the PlanetXbox360 feature article:
"Going into E3, we all knew that the next chapter in the Halo saga would be dropping this Autumn in the form of Halo 3: ODST. What we didn't know, however, is that bundled with the game would be an invitation to play the multiplayer beta for Halo: Reach, a new title announced by Bungie Studios at Microsoft's press conference earlier this week. Beyond the official trailer, little was said on the details of this new chapter in the Halo universe, but the message boards and blogs have been hard at work trying to piece together any further details regarding the game. The first and most obvious of these details comes from the title. Halo: Reach is most likely referring to the Covenant attack on the planet Reach before the events of the first game. Author Eric Nylund in his book Halo: The Fall of Reach has already fleshed this storyline out. Evidence to support this assumption is all over the trailer, which shows a planet being attacked and the words "Halo: Reach falls 2010." Going from this tidbit of information, we can now place it in the timeline for the Halo universe."