MacNN credits an undisclosed source as reporting that Apple has signed a lease to the former Brasserie Perrier restaurant at 1619 Walnut Street [Google map]. This is a protected building with historical significance, so there's a limited amount of changes that can be made to its facade. Of course, the Apple Stores are known for their dramatic glass storefronts, though more subtle facades have been used in Europe.
AppleInsider reports that the retail market has grown in that area recently, specifically across fine dining restaurants, shops and banks. Apple tends to build retail stores in this kind of market.
Good luck, Philadelphia! We're hoping you get your first store soon.