Call HTC support,they will issue you a new one for free, under the warranty your allowed one accessory per unit, the back cover falls in that category. That's what they did for me. I now have a new one I use when I go out and a scratched one I use at work.
AT&T Black Jack>AT&T Fuze>AT&T Tilt2>T-Mobile HD2 on AT&T!
HTC HD 7