QUOTE(Bungie.net weekly update thingy)
Q: Who is doing the banning Xbox Live or Bungie?
A: Bungie is banning players from matchmaking who use modified content. We're only banning you from matchmaking on Live. And we're only banning you from Halo 2 matchmaking. You can still play arranged games and you can still play your other games on Xbox Live. The Xbox Live feedback system will continue to operate as normal which means you can have your account suspended for the usual bad behavior continual cheating, foul language, etc.
So, as you can see, your xbox won't be banned. Even if they detect that you have moddified maps, all they'll do is bann you from Matchmaking.
Stop freaking out people!! we're talking about bungie here!
Edit:Just in case we have haters reading this, I, by no means, am trying to say that:A ) I have modded maps and that I cheat on Live.
B ) You can have modded maps and cheat on Live.
C ) You won't get banned from matchmaking if you cheat on HALO 2. (If you do, I hope you will get banned)