The best method I've heard for cleaning really dirty vents is compressed air. Depending on the sources available, you should be able to blow most of the loose dust off the sides of the ventilation system. Even a can of compressed air from the local office supply store is better than nothing.
Obviously, depending on the configuration of a particular car, you may have to remove the dash, etc., to get direct access to the air path. If you're using one of those small cans, you're definitely going to want to open things up (they don't have a long reach).
Once the really loose stuff is blown away, you can fashion a chimney brush-equivalent from a stiff wire or piece of dowel tied to a small rag. Be certain that it's tightly affixed, though, as you don't want to leave a hankerchief jammed down in your HVAC system....
Be aware, you're going to shake loose a lot of dust. I strongly advise wearing a dust mask (a full ventilator is probably not necessary), working outside with the windows open.