Today on EBGames.com, has a list of games showing that a Halo 2 expansion pack is in development.
The Halo 2 Multiplayer Map Pack is a collection of nine multiplayer maps, as well as non-playable video content (a cinematic and a mini-documentary). Delivered on a standard Xbox DVD disc, the multiplayer content can be used by gamers on Xbox Live as well as those who play multiplayer via System Link or via Split-screen (on one Xbox).
It was first announced yesterday on the official Korean Xbox website. More information will be stated as Xbox Advanced receives it; stay tuned.