In my random meanderings on the web tonight ran across this 55U rack: http://www.server-racks.com/how-tall-can-a-server-rack-be.html
There's a real problem that's missed there. Space in data centers isn't based on how much equipment you can fit in a rack. It's based on power consumed by that rack. Most data centers today are designed to accommodate about 150 watts per square foot*. A rack in a perfect hypothetical environment consumes about 16sf (6sf for the rack, 6sf in front to pull out/install machines and 4sf for rear access.) In the real world where there are walkways and clearances and pillars, that rises to about 25sf per rack. 25 sf x 150 watts = 3750 watts/rack. Then do your own math.
At some point you need to take a holistic view. If your rack is consuming 10kW the two racks on both sides of yours can consume .. how much? 650 watts each??
* some DCs are built to 100w/sf, some claim 1500w/sf, please ASK your provider.