[CS3] Flash Fog Of War Effect

Hey guys i'm struggling with this one and cant even get the concept off the drawing board.

The idea is this, I have a map (a simple large image) and I need to cover the entire thing with fog.

However as places are discovered by the person I need to show them the locations (and paths to the locations) they have uncovered.

Originally I did a simple mask but the fog of war effect looks really bad as you cant do semi transparencies on masks.

So in short I need a map that can be scrolled (just move the movieclip around the screen) while the areas visited will be shown but have a fog effect around them (so the town / place would be seen and then the view would fade out with a moving fog effect.

I can move the movie clip around not a problem and I even found a nice fog effect which at present I cant get working.

anyone have any ideas???