Call of Duties community czar Robert Bowling revealed via twitter that over 1,600 cheaters have already been banned from online play in MW3. He is quoted as saying "any attempt to cheat, hack, or glitch in will not be tolerated."
Regarding the length and severity of any bans issued he said they are determined based on the level of "douchiness" from the cheater. And although some bans are not permanent, in serious cases some are.
"Reporting [cheaters] in-game through [Modern Warfare 3] goes directly to our security team, who issues the bans directly after verifying," Bowling noted. "Every ban unique to the level of douchiness of the offense. The greater the douche, the greater the length. PermaDouche possible."
So if you see any cheating in game be sure to report it, the reports do not fall on deaf ears.