Cheating is the death-knell for many multiplayer games, and as much fun as it is to fly around the level firing lasers from your crotch, that sort of nonsense just isn't tolerated.
Thankfully, Infinity Ward has been on top of swiftly banning offenders, with another 1600 on the way out as of today, as reported by Modern Warfare's Creative Strategist Robert Bowling.
@fourtwozero – "Any attempt to cheat, hack, or glitch in #MW3 will not be tolerated. 1600+ bans issued. Updates in works. Please cont. to report offenders."
Not reported is which particular hacks are being cracked down upon, though honestly 1,600 players banned doesn't sound like much of a dent in the problem now that the game has sold 7,000,000+ copies. We're assuming there's more than a few cheaters still lurking out there. So be on alert, and remember, if you see something, say something!
(That means actually report them by the way. Don't just berate them over your headset. Actually, do both.)