‘nuff said.
Luke Smith’s confirmation:
http://www.neogaf.com/forum/showthread.php?p=7249107#post7249107
IGN:
http://xbox360.ign.com/articles/809/809569p1.html
Joystiq:
http://www.joystiq.com/2007/07/31/its-official-halo-3-has-four-player-online-co-op/
Some other site:
http://n4g.com/xbox360/News-55843.aspx
Currently Playing: Guitar Hero: Rock the 80’s