The only time I've ever seen this happen is when my friend was struck with a sudden case of "the dumbass", and thought it was a good idea to move his console with Gears of War playing. Needless to say (but I will anyways), it got a nice big scratch around it.
Since I've never seen it happen for any other reason, I'm gonna have to say that it's not Microsofts fault if you can't take the time to skim the instructions for giant red letters saying "DO NOT MOVE WHILE PLAYING".
Besides, they sell disk-scratch repair kits that work quite well for about 20-30 dollars anyways.
And this, my friends, is why I think a modchip is an integral part of any console. It's bad enough that you have to pay 4 times for the same game if you want to enjoy it on 4 different systems, you also have to pay double, triple and more for every game if someone, including Microsoft, damages it. "Pirate and try out everything you want, then pay for what you like and want to see more of" is my moto.
I've had my 360 for about 14 months, it scratched two games in the past two months. I didn't even have to move it! The fact that Microsoft isn't refunding scratched games is ridiculous. Well, they might refund my copy of Halo, but the only downside there is that I don't fucking play Halo.