Getting it replaced at Apple will work or you can search eBay for different colored or patterned backplates if you want something different. I've ordered an aluminum one and a white one and both have worked great.
The scratch shoudnt affect your picture quality at all. This was a top on here some months ago and there was a link to a person who tested this theory, using a shattered fish eye lens next to an un shattered, almost impossible to tell difference in quality.
This isn't the one that was presented, but is still interesting.
http://www.petapixel.com/2011/06/16/how-dust-and-damage-on-lenses-affect-image-quality/