This game, Halo 2 Vista, will follow the same rule that we had for Halo 2 Xbox. There is to be no discussion of modding in Matchmaking, as well as no discussion of any types of cheating mods. While I'm not sure on the specifics of "Custom Games" for H2V, assuming they will be the type where you can play your user-created maps, that discussion will be fine (In contrast to Halo 2 Xbox, where we do not support any modding on Xbox Live).
This will probably be updated when I or the rest of the staff is positive on how the Live system will work in the game.