Reported in this forum post on the official IW forums and our very own forums apparently hackers have found a way to go around the IW.net system and send a Trojan virus through the IW.net system to Modern Warfare 2 players. Reverse engeneering the source code for Modern Warfare 2, the hackers inserted their Trojan and used the IW.net system to spread it to players across the globe. This was brough to light after several players got notified of a breach in their security by their antivirus software. The Trojan TR/Crypt.XPACK.Gen
The Trojan would exploit the port that is opened by Call of Duty when you are a listen server (when you are the host of a game) to send you the Trojan without your knowledge.
Sauce to read more.
I lol'd.