Hi all. My Phantom 1.1.1 has the same problem with the two rear motor mounts. I've found a solution for this as re-shelling is not so cheap I decided to fiberglass the two rear motor mounts with 3 layers of 18g/m2 fiberglass cloth.
I also posted a video on YouTube:
http://youtu.be/2TgdkMgMgaU