I have run a 400 GPU farm with absolutely no A/C. If you need help, please contact me on skype (Kotarius). I can save you a lot of money.
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.
A properly designed mining rig can be quite comfortable at 90 F ambient, and most should be able to handle 100 if enough space is allowed between the GPUs for proper cooling.
ASIC rigs vary - some handle higher temps well, some need to be downclocked.
Even the big boys have started realizing this - reference the Yahoo "Chicken coop" data center design for a well-known example of a MODERN efficient data center that does not use traditional A/C at all.