Most roofing nails are rather short. Take a careful look at it and the position it's in. If it's short enough and angled right so that it didn't penetrate through the rubber into the cords, the damage is probably negligable.
If it did reach the cord, there is about an even chance whether the tire has had enough damage to be dangerous. At any rate, if it holds air with the nail removed, it should be safe enough to ride at what regular people would consider "normal" street speeds to the tire shop.
