I've been able to play for many weeks.. I didn't change anything on my side, no router modification, nor internet provider, didn't hacked the damn game..
Now.. it's been 4 weeks I cannot connect online... all I receive is a "The Mw3 server is not available at the moment, please try again later, or visit www.callofduty.com/mw3/status to obtain update (this is a free translation, but i guess this is about right)
I did check the installation file, reinstall the whole game, even connected my gaming station directly to the Internet modem cable (who is not a router)
Even on the website, we can choose between PS3/XBOX360/PC and when I click on PC.. i got a "Sorry, an unexpected error occurred." when loading the http://www.callofduty.com/mw3/pc
So... no answer from Activision since I opened up a case on the support section of their website...