I don't believe the servers have been down today at all.
Try switching off your modem/router for a couple minutes, then switching them back on.
If you still can't connect, you may need to follow these steps to make sure the ports MW3 uses are open:
http://portforward.com/networking/staticip-xbox360.htm How to set up a static IP address on your Xbox 360.
http://www.portforward.com On this webpage select your Router brand and model > Select your game
Disable any firewall or security features on your router.
Set port forwarding on your router to the IP address of your Xbox 360. This game uses port 3074 (UDP/TCP).
Additionally Xbox Live requires ports 80, 53 (TCP) and 88, 53 (UDP).