Apparently with todays release of the Resurgence map pack on Modern Warfare 2 Steam forgot the include the maps in the map pack.
According to the MW2 forums on steam here
Everyone has been having this issue. In order to fix it you have to deselect auto updates and then turn them back on. This will prompt steam to redownload the update. I would considerer forgetting the maps in as map pack to be a complete and total fail. Insert fail sound!
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