Every source I have found says that if the outer fabric of a down pillow cover is worn, the pillow should be replaced. However, if your pillow is a Scandia brand, they do have services for restoration. You can find out more here: http://www.scandiadown.com/cleaning.asp
There is a service called Blanc Plume which is a French laundry. They also do restoration. You can contact them through this website: http://www.arrow-care.com/blancplume_faqs.html Their phone number is XXXXX the bottom of the page.