http://quake2world.net/blogs/jdolan/gpl-quake-anticheat
Gives an interesting overview on a potential open-source implementation of such an anti-cheat feature.
Personally I am in the camp of "it is only a game and you can quit if someone cheats"... but sometimes there are only a few servers active, and one guy (edit: or gal ) can ruin it for the rest of players... so some sort of anti-cheat tool might be a good thing after all. And for sure it is something many players demand and it is also used as an argument against open-source multiplayer games (as silly as that might be).