Well actually every game has modders/cheaters to some degree. Halo 2 is by far the most popular game for XboxLive since it's start. So therefore the number of creep-o's that crawl out of the woodwork to cheat goes up as well. I would say that from a ratio standpoint, XboxLive as a whole would be about 15% cheaters, and that for Halo2, that ratio increases to closer to 25-30% cheaters.
Plus with the matchmaking system, which is a great idea, you are constantly matched with randoms. So the chance of encountering a cheater is higher. When I played Mechwarrior a lot, I never encountered many cheaters, because I would play games with the same people over and over. If a dude cheated, you had the choice not to play with him. And without the rankings, people did not have as much incentive to cheat.