I did not notice this on my RAZR, but I realized that the back of my RAZR Maxx has some waviness to it--more pronounced at the top, but it also can be felt by the center portion of the kevlar backing.
I don't recall feeling that when I first got the phone. However, the phone's back did get pretty warm several times--I'm thinking that was the cause for this.
You can see a little bit of the waviness due to the shadows in the pictures. The curve towards the top of the phone is the most apparent.
Are your Maxx's like this as well?