I recently built my theater in a walkout basement, but 3/4 of my theater room is thick concrete slab walls. I built the stud walls out about 6" from the walls and filled the cavity with insulation. I chose a thicker shag style carpeting and used the thickest carpet padding available. The sound is amazing and the concrete walls behind the drywall and studs does isolate most of the sound to inside the room, which is great for the neighbors and my wife! Bottom line is I wouldn't worry about the concrete walls...you should have no problem building the theater of your dreams!
Take a look at my first pics in the Axiom gallery (Skubics Home Theater). They were just uploaded this morning so it very well may be the first picture you see in the "Wall'O'Fame" box to your left.
