It's now available on the Xbox Live Marketplace. It costs 800 Points, around $10. It includes three maps, Rat's Nest a HUGE underground maze complete with several vehicals including a docked Pelican, Foundry a small indoor map much like the Pit and Standoff an outdoor map with several bunkers and strongholds, it's by far the biggest map in Halo 3.
These maps are not available in matchmaking, as Bungie is going to split thier players into two groups, those with downloadable content and those without. EDIT: Bungie just created a new matchmaking Playlist for the Maps, Heroic DLC.
That's all I had, if anyone has downloaded it and wants to play a custom game, my gamertag is DIRTYDEALIN
Send me a message.