Z team (or Zealot Mapping Team) has been doing a Halo 2 Mod for Halo CE.
They have released two public betas (Coagulation and Sanctuary) for their Multiplayer. Their single player will just be a mod of the original campaign with the Halo 2 weapons\bipeds\vehicles,etc. It is not available yet and they have not set a release date yet for it. Z team's pb2 thread has become sort of a discussion forum so just stop by there sometime to get updates.