Warming Up The Forge
Tyson "Ferrex" Green has written up the first part of a three-part series on working in Forge, Halo 3's in-game editor. You'll apparently be able to call on a startling variety of objects in the editor; although you won't be able to change map geometry, you'll be able to add objects so large that they will, effectively, change the flow of the map.
And, of course, it's not just editing, it's also a play mode: