Yea i've had similar, i just restarted steam, and Disabled and re-enabled my Network adapter, seemed to have sorted it. A restart sorted it for Ghost_Rain i believe.
Unfortunately MW3 isn't made by IW.It's the remains of IW after all the talent left and to prop up IW's sorry bunch of tea boys they have recruited Raven Software and Sledgehammer Studios. It may turn out to be a great game but my worry is multiple studios don't make for a good game - just look at Medal of Honor.