While the date has been rumored for quite some time now within the gaming press community, Bungie finally made it official. Halo 3 will be launching on September 25th in North America. The game will (obviously) be accompanied with huge marketing campaigns, including tie-in products with Mountain Dew, the Microsoft Zune, many retail outlets, and more.
Microsoft is touting the upcoming release as the most anticipated game release of all time, and they may be right. However, I think with the smaller install base of the 360 as compared to the original Xbox when Halo 2 released, it would be pretty shocking if the game sold as much (or more) as Halo 2 did on its launch date.
Halo 3 releases one day later in Europe.