I've not seen any other mock ups of Mac Pro designs, but I really like the one you've just posted – very in keeping with the new iMac. But wouldn't glass be impractical for a machine that gives out lots of heat? Plus I'd miss the handles (unless they got the weight down).
actually..glass conducts heat..its up there with aluminum..i believe aluminums specific heat rating is like 867 and glass is 710 or something like that. so..glass is its self, a conductor of heat, thats why windows are double/triple paned the AIR inbetween the layers of glass is acting as the insulator.
unless..of course u meant there are no see able air vents, for that..well..there has to be, no way can it stay cool enough without some fans pushing air through it.