ORIGINAL STORY: We'll likely find out whether this is a big misunderstanding in the next few hours, as the Microsoft E3 press conference is set to begin in less than an hour, but for now let the rumors run wild. While we can certainly expect a look at Halo: ODST during the conference, a slip-up on Bungie's forums may have inadvertently blown the lid on another segment during the conference -- the announcement of a Halo game called Halo: Reach. Website Skoar (via Kotaku) managed to capture a picture of a sub-forum dedicated to Halo: Reach, as well as some cached discussions about it that have since been removed from the forums.
For those who don't pay close attention to names, Reach is a planet in the Halo universe that housed a major human military force prior to being largely destroyed by the Covenant. The very first Halo novel, The Fall of Reach, covers the annihilation of the planet which took place just before the events of the first game, Halo: Combat Evolved. Halo: Reach could potentially be a prequel or even the name of Peter Jackson's Halo project. Perhaps Halo: Recon's name was changed to Halo: ODST so that it wouldn't be so easily confused with the name Reach? Might this be the rumored Halo mobile project?
Whatever it is, we'll be able to debunk or confirm this story very soon.
E3 2009 -- the game industry's most important convention this year -- is happening all this week! Check out E3.1UP.COM for news, previews, podcasts, videos, blogs, and more direct from the Los Angeles Convention Center.
UPDATE: So much for the mobile Halo game theory. Xbox360A.org (via Kotaku) has noticed an Achievement page listing for Halo: Reach. There aren't any actual Achievements on the page, but the page's existence lends further evidence that Reach is real.
The games coming soon from bungie:
Halo 3: ODST (They did say this one

) (Fall of 2009)
Halo: Reach (Multi player beta) (The one bungie didn't want YOU to know!) (Sometime in 2010)