Middle Atlantic has switched rack shelves to a new style that is not enclosed on the sides anymore. Instead, they use a cut out steel support system which leaves the sides fairly wide open, as well, there are venting holes all across the bottom of the shelf.
Your worries about ventilation should be for ensuring that the closet itself is not completely sealed and trapping a ton of hot air within it (Middle Atlantic can help with that as well!).
But, if you go to their main webpage at
www.middleatlantic.com - you will find that right on the front they have a flash animation that shows the design of the new rack shevles.
IMO - you still want to design the rack to leave a full rack space between each component if at all possible. Use VTF spacers between shelves, and include serious ventilation around the rack to remove hot air buildup inside the rack.