The Modern Warfare games have been rife with cheaters online since their release, and nothing appears to have changed in Modern Warfare 3.
Despite assurances from Activision that the company would be more rigorous in its handling of cheaters with the introduction of the Elite subscription service and a studio dedicated to its online service, cheating continues to persist in the online modes of Modern Warfare 3.
If it isn't boosters teaming up to exploit the game's experience point system, it's a modification that allows players to reward each other with millions of experience points in a single kill, allowing players to rocket through prestige levels in an instant.
A couple of cheaters put together a video detailing exactly how the whole thing works. Judging from the video, it seems they also like Skrillex.