According to Xbox.com, Halo 3 has gone gold, or been "released to manufacturing" according to the article.

Looks like RT mentioned it first in the HBO forum, and of course it's been picked up by HBO itself, Major Nelson, Xbox 360 Fanboy, GameSpot and the horse's mouth: Bungie.net.

In a short but sweet news post, Luke Smith writes:

We have to say thanks to everyone at Bungie who worked so hard to make this the best game we've ever made. We have to say thanks to everyone at Microsoft who helped us make it the best game we ever made. We have to say thanks to our tireless team of artists, designers, engineers, musicians, technicians, writers, directors, producers, thinkers, administrators, ninjas, specialists, gorillas, webmasters and pimps. We’re a family and we made this game for you, our extended family. We hope you like it. Nobody deserves any more recognition than anyone else, but an extra special thanks goes out anyway, to the test team who got it to this stage, in a pool of their own sweat, tears and blood.