Bungie today announced that Halo 3 will be shipping in the US on the 25th September, followed a day later with a release in Europe. According to Bungie "Worldwide dates may vary slightly depending on local rules and conditions." Halo 3 was always promoted as coming out in the Fall 07 but few expected it to be just two days into fall.
In a Q&A on Bungie's website Jonty Barnes, Executive Producer said that announcing the release date changes nothing for everyone at Bungie, "that's been the plan for months and months. The only thing it does however is make the end seem very, very real to people. To be honest, the critics in the studio are those who drive and stress the team the most. External dates and announcements just bring home the excitement and expectation the public has for the game, and this makes the hard work feel worth it."
News source: Bungie.net