It does make sense that various security agencys, and terrorist groups and such would have some practitioners on the payroll.  I don't know about a special black agency, but you would think that CIA, MI-6, ect. would want those kinds of assets if they could get them.  Plus, a lot of the things Dresden has run into just might be considered national security threats.  I was thinking of playing it so that the White Council has kind of a secret treaty with the intellegence services.  It would state that the wardens don't mess with lawbreakers who are acting for an agency.  They get a pass on killing, messing with people's minds and things like that.  The Council probably would not want to tick off that particular bunch of mortals, as they are much bigger and could likely wipe the council out even without any kind of majic.