Hi eugeme,
Ipad back cover is very delicate, once it gets scratched you will never be able to restore it the way it was.
You can get the back cover for a reasonable price and replace it yourself following guides on this site.
In case you dont want to do that, i have tryed a bunch of things to try to make the aluminium look as neat as it was before. Te best thing i found is the aluminium cleaner (the guy from the hardware store told me it was a cleaner) and it worked really good. It looks like an ugly soap bar of brown color. At first i thought it was too rude, so i decided to give it a try "cleaning" my bath room or shower aluminium sliding doors. Thath thing did magic on it. Contrary to what i thought, the "cleaner bar" started to wear away leaving the aluminium clean and smooth. I realized that a long aluminium piece screwed to the wall its nothing like an ipad or iphone 2g back cover, so i decided to work on an iphone 2g first. It worked really well, i didn't do as well on the ipad since it is a much larger area i guess that if i get to do it much i would get the hang of it.
I hope this helps, either to make your ipad look better or any aluminium door or frame in your house = )