I have a ZUM plastic cover on the back of my wife's iPad. The Smart Cover's hinge and track attaches to the ZUM cover, not to the side of the iPad. Since each hinge has a very small plastic spacer that prevents the metallic portion of the hinge from touching the iPad's aluminum frame, I haven't been able to find any damage whatsoever on this iPad.
But in my view the Smart Cover is too heavy, and instead of having an aluminum track and hinges that are exposed, these parts should be designed so they are covered with a layer of hard plastic. Also, the Smart Cover should not latch to the iPad's aluminum case. It would be a lot cheaper for Apple to make Smart Covers that are mounted on some kind of back cover instead of the iPad itself.
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I also bought an iPad 2 "fake leather" case, which in my view is not only cheaper than a Smart Cover, but better since it covers and protects the front and back of the iPad from shock and bumps. The front cover folds like the Smart Cover, serves as a stand, and it also turns the iPad on/off.