I don't think I'd use acetone. It has a bad habit of melting plastic. If you think you must, test it on the inside of the back cover first and see how it goes. That way if it does melt, at least you wont see it. Maybe try the same thing with WD40.
Try some MEK (Methyl Ethyl Ketone). It gets almost anything off. Although I'm not sure about what it will do to plastic. I usually just soak a cutip with it and clean stuff. Try it on the inside of the back cover.