I haven't really kept up with pre-launch Halo 3 news lately, and today I came across two nuggets of H3 goodness:

1. The Elephant. I don't know any specifics at all, other than a) it is a vehicle and 2) you can fit 16 players + some small vehicles in it.

The Elephant

2. Forge. There will be a map "editor" called Forge. You can't actually design levels with it, but you can highly customize weapons, vehicles, and items within the maps. e.g. create a giant pile of fusion cores or something.

I found both of these pieces of information in this article.