Here are the Sharpie website's tips on removing marks from fabric:
http://www.sharpie.com/enus/pages/faq.aspx#Q13
Since alcohol is mentioned as a solvent for the permanent ink, I'd probably start with that first -- testing on a scrap of the fabric, not on the actual quilt! If you have the extra fabric, I would use the same Sharpie marker on a test scrap, let it dry, then try to remove.