Microsoft announced today that they would cease all support for Xbox games, be they disc-based or purchased via the Xbox Originals service, as of April 15, 2010. The reason for this cessation is that Microsoft has changes planned for the Xbox Live service that they would be unable to implement if they continued supporting Xbox games.
For gamers, this means any online gaming you plan on doing on Halo 2 or any other online-enabled game will need to be done soon. All offline modes will continue to work on the Xbox 360, including offline multiplayer and single player modes.