The most recent update to the Bungie website has revealed two new maps, a vehicle, and armour permutations for Halo 3.
The first of the new levels is Last Resort, a "spirited remake" of the Zanzibar map from Halo 2. It's largely the same, but should provide a refreshing experience thanks to a larger walkway and bigger interior section with multiple entry points. Don't rise to it, keep it cool. You know what Tom said last time.
The other map is made up of lots of external hallways leading to a multi-level interior featuring a central theatre. It's called Epitaph and it has undergone several revisions since early internal beta, so chances are it will be different to what you may or may not have seen already.
Also included in the update were new armour unlockable armour skins usable only in multiplayer, and a Brute Chopper vehicle for the Spartans - featuring four cannons that all fire together in quick succession, and a top speed of 75 mph.