Halo 4 is now available on Xbox Live Marketplace. The War Games Map Pass (2000 MSP) grants automatic access to it and two other upcoming map packs, otherwise you have to stump up 800 space marine bucks. Those with the Map Pass can access the pack through the Halo 4 in-game store, and it's expected to come there today.
The Crimson pack includes Wreckage, Covenant, and Shatter maps, along with a playlist featuring the trio exclusively. Today also sees Extraction come to Halo 4, a new multiplayer mode which is all about defending and attacking beacons planted by players on the map.
Update: Via two tweets, developer 343 Industries apologized for Map Pass and Limited Edition holders being unable as yet to download the map pack. The tweets read: "We are aware of the issue in which Map Pass and LE owners are not being recognized to download the Crimson Map Pack at no additional cost. We are working on the situation now and will provide an update as soon as we have one. We apologize for the inconvenience."
Update 2: Microsoft's Xbox 360 Service Status page acknowledges the problem facing Season Pass holders, writing, "Season pass holders are you unable to access your Crimson Map Pack download on Halo 4? We're working hard to get you into action in the new lands! Thanks for being patient. We'll post another update in 30 minutes." That was at 7:30 a.m. PT.