Careful with verifying game files through Steam.
 

As some people may already have noticed, verifying CoD's game files consumes an activation of the Solidshield DRM. I can't believe that this was really intended, would be nice if somebody in charge would take a look at it.

For people who don't know about the verify option yet:

Steam's "verifying game files" option comes pretty handy, since it checks all the games if you should encounter troubles with your game and replaces files that have been broken for whatever reason, preventing you from having to download the full game again.