At Gamestop.com you can pre-order all 3 versions of Halo 3. Gamestop is also taking pre-orders in their stores. After clicking the url, click the version you want on the right side of the screen.
The release date it set for 11-01-06
Regular Game: $59.99
Collectors Edition: $69.99
Legendary Edition w/ Helmet: $99.99