Audiobahn and kicker both make subwoofer grills. They are more decorative but will provide some protection. Two of my friends had the same thoughts and one bought a grill off of ebay and the other just reverse mounted his subs(alittle more protected, but not much).
Pretty Much. All you need to do is screw the cover onto the enclosure. A sub is fairly strong. But if they are hit with a sharp object the conoe can be pirced. A reverse mounted sub is not as vonerable. But the subwoofer grill will add to the protection of the sub emencly. And if the subwoofer is playing and an object is leaning up against the sub the grill will prevent the sub from being hit or damaged.