jeffhaag,
Considering the investment going into that size tank, I wouldn't take chances with the stand. Overbuilding it will help spread the loads out better. For the 36" width, consider placing stringers running front-to-back in several locations. Galvanized deck joists hangers would provide a lot of support and with a stringer say every 20 inches, the tank bottom will be well supported. Nice long decking screws into the ends of the stringers will keep things from twisting. Thats how I would do it anyways.
DrkPhx,
The short answer is no, it doesn't need a skin. However, a skin will add a lot of stiffness to the stand. For an in-wall application, I would just make sure there are enough screws between the legs and the screw strips to keep things square. If you are worried about it, a couple plywood corner stiffeners wouldn't hurt but since its in a wall its unlikely to be pushed sideways by someone running into it.
For those of you building stands, keep the pictures coming. Its always interesting to see the many variations people come up with.