I have the exact same phone, but don't have that problem. I've read that some cheap, eBay cases might cause similar staining. Where did your case come from? Have you tried rubbing alcohol with a q-tip??? In a small area first, of course. Also, a mixture of bleach and water applied with a q-tip might help. I'm totally just throwing ideas out. While I don't think rubbing alcohol or diluted bleach will damage the shell of your phone, I've never actually used either. Hope you figure out a good way to clean it up soon.