The Magic Mouse has not been around for even a year yet, so it would be hard to evaluate it based on whatever someone has seen with the Mighty Mouse. Alan in T.Dot, I think you're confusing the two different mice. To answer the question, I've had two Magic Mice since just about Day One and have had no problems with scratching or discoloration. For the record, prior to that I used Mighty Mice, both wired and wireless, since their respective Day Ones as well, and again, no scratches or discoloration.
Right now I'm trying out the Magic Trackpad, which I was finally able to get into my eager little hands today......