The new Halo 2 maps from Bungie will be available for public comsumption on April 29th. The 2 immediately free maps, Containment and Warlock will be downloadable thru the Xbox-live download content menu item while the Turf and Sanctuary maps will be available for $5.99. If you're patient (like I am), wait until June 28th and all the maps will be freely available. See the article straight from the horses mouth here.
as a bonus, check the Warlock 3d-Quicktime screenshot here.