i've had a modded xbox since 2003 (the same one i've always had) and been on live for a few years, too, and i've never experienced cheating (never really played halo 2 except in the early days). i can't imagine this hacked firmware will spell doom for my online streetfighter-ing, but still.
here's a question, though - if someone was to hack a game and then burn it, wouldn't that mess up the signature? if so, you really could only play proper 1:1 backups on live and hence would be no more likely to cheat than those owning an original copy of the game?
besides, what with the action replay on the xbox, that'd be the first port of call for cheaters worldwide and that's been sorted out somewhat.
i think this is good news purely from a scene progress point of view than as something i'm actually going to seriously look into or attempt myself, but we shall see how this affects live.
i'm with the rest of the posters on here in that it probably won't do much, if any, damage - halo 2 cheaters have already caused major damage and they were using softmods, etc, so i guess it'll be the same...