About the Scratching of Discs
If you insert a CD into the side loading DVD player slot there is a good chance that the disc will become scratched (on either the front or back). This problem affects both the 21.5" and 27" models (although the severity of the scratching may vary from machine to machine). Apple does not consider this to be a design defect but rather the result of user error in not inserting the discs carefully enough.
Examples of Scratched Discs
Fixing the Problem
To minimize the damage try to pull the disc towards you as it loads so that the back does not get scratched. You could also use a Dremel tool to grind down the aluminum edges.
Another solution is to put packing tape along the front and back of the CD slot. The photo displayed to the right is an example of the tape "repair" performed at an Apple Genius Bar.
Special thanks to Chris K. for supplying the photos and some of the info on this page.