What I want to know is, when is the sheer weight of all the upper Macs going to crack the plastics of the lower Macs and bring the entire mountain of machines crashing to the floor?

Seriously, it would be more prudent to put each row of Macs on its own shelf. I suppose a few rows of Macs stacked atop each other wouldn't hurt for long term storage. But count how many rows there are in that photo! I would be very, very afraid to take a step in that room, more less sit at the desk shown in that photo. Scary!