According to a post on Bungie's official blog, Halo: Reach is totally finished and ready to go... but it's still not coming out until September 14.
"A few days ago we released Halo: Reach to Xbox Certification," posted executive producer Joseph Tung. "Assuming things go well in certification, that means the only things standing between you all and Reach are the manufacturing process and shipping the discs to stores. It’s been a long haul getting the game to this point but the team is incredibly proud of the game and can’t wait to play with everyone on September 14th."
Source: Bungie's Blog