http://forums.steamgames.com/forums/showthread.php ?p=27001288I figured out how to resolve this issue.
Go here:
C:\Program Files (x86)\Steam\SteamApps\common\call of duty modern warfare 3\zone\english
I moved the patch_mp.ff file out of that directory, and then verified the integrity of the game cache.
It downloaded the file, and was able to play without issue after