I'm quite curious how this is done and what climate you're in that no AC is used.  I live in the Midwest, United States and in the summer, the ambient temp will far exceed what you want a room full of computers to be at.

I'm having my HVAC company come over next week to consult on adding a supplemental AC/system to only cool the server room(s).  If I can do this with mostly moving air and a little cooling (in the hot summer days), that'd be ideal.  If I had to choose between shutting down the farm on those hot days and paying for electricity to cool it, that's an easy choice.

The "pressure" aspect is interesting, I'll definitely do some additional reading on that. 

Good stuff guys.