did not know how to directly confront your situation. You and I know that it would be soo much easier to just pad the slab and then attach the perimeter tack strips and then lay the carpet, but with a basement you are wise to address the moisture that is inevitalbe to come. The plastic sheeting is wise, but it will not work because you have to secure the ply to the slab and that will compromise the sheeting anyway. You might want to by pass the sheeting and use that water proof weep proof paint that will seal the slab from moisture wicking. But I am not sure it will completely work.
Chuck, the ply wood must be secured to the slab and secured well. If not the ply will warp, twist, and roll and all will be seen in the carpet and the carpet will wear at those irregular spots.
You might consider if you have the space to install treated furring strips to the slab and then screw the ply to the nailing strips. This will give you some breathing room and allow something for the ply to nail to. I would go every 12 OC. The furring strips must be secured and glued to the slab. A screw every 8 inches or so and then Liq nails for concrete on that.
All fasteners must be galv or stainless.
This is allot of work and will make your ceiling shorter.
Why not lay down a heavy poly mat and then put some good quality in door out door carpeting down, or a good quality poly carpet? This would be much easier. Be sure and seal the concrete if you do this.