I spotted this on the official Call of Duty MW3 forum:-
"Hey everyone -
A quick update for you regarding the hacking issues in MW 3, we'll be releasing a patch either end of this week or next week which will change our servers to P2P. This will help Steam detect whether the players you're hosting or joining are valid players.
This change may cause matchmaking to take longer so we ask that you have a bit of patience when looking for games. If you end up having major issues or lose the ability to match at all please let us know so we can provide a fix for you.
Thanks all!
Side note: If you do run into hackers and would like to report them, please send their info to Valve. Here are instructions to help with that:
1. Open the Steam overlay 2. Click on "View Players" 3. Find the player in the "Current Game" list 4. View profile 5. Top right corner click on "Report Violation" 6. Tick suspected cheater 7. Put information in there and Submit the Report".
I'm not sure of the implications and what this patch will actually do, but I think it may be aimed at those who host modded lobbies? There's also a discussion about it on the steam MW 3 forum too. Any-ways this may also mean that with DLC 4 being released today, the game may get patched twice in the space of the next 7/10 days???