There is really no question about it, people mod halo mp maps on live; the new maps more often, but even the older maps occasionally.
The way most jack asses do it is with the new maps that are stored on the Hdd. Once the map has been modded, and signed with the appropriate key, then the mod chip is turned off, or the softmod is undone. Now the retard can go on to live, and all the new maps will be recognized as authentic ones. The ding dong will not get banned for quite some time, contrary to what bungie has said, and most think. He can turn off his xbox and log off live, and then return in all is pathetic, cheating glory for many a day before bungie has enough "proof" that the said retard is cheating.
The other way is to log onto live with your mod chip on, or softmoded xbox. One can do all the cheating one want with the old and new maps without being booted/banned until he logs off live and tries to rejoin, then he will be banned.
EDIT: If i play halo2 matchmaking on live for but 2hrs at a time, i will play against someone with modified maps. Because of this i now only play non-matchmaking games on live, and mostly xbconnect.
Halo 2 really needs to have two catigories of games, non-ranked and ranked......both with the same amount of game types. Plus, a section of nothing but MLG with different game types to choose. Just having Hardcore, with all gametypes randomly mixed into one, doesn't cut it. I just want to play TS MLG.