According to the post in the IGN forums, there are 4 new maps available for Halo 2 Multiplayer via the Xbox Live Download Center.

Two of the maps, Containment and Warlock, are free. The other two, Sanctuary and Turf, will cose you $5.99.

From reading the posts in the forum topic, looks like they are on a rolling release schedule, as they have shown up at different times for you users in different areas.

Link goes to IGN topic.