Sept. 25th. That date sounds like a long ways away for me right now, but I'm sure it will arrive soon enough. That's the announced launch date for Halo 3. For those who didn't play in the beta, Halo 3 is more than just a graphical upgrade to the Halo 2 engine, it makes advances in game play aspects and balanced play.
At any rate, I'm anticipating a large viral campaign for this, so keep your eyes open on the Halo3.com website.