Anyone ever heard of a datacentre using equipment other than air conditioning, I haven't.
Water cooling was much more common in early computing history. As the power density of a rack fell below 5KW it became easy to cool with air. However power density are rising and for high performance computing (which can easily achieve 8KW/rack +) water cooling is starting to come back. As someone else said just search google. Many supercomputers are water cooled because they have high power densities.
Mining blows those densities out of the water (no pun intended). A 4U case could hold 8 GPU. 1200 to 1500W per system. 11 systems per rack = up to 15KW/rack. Ouch.
I see people running custom cooling solutions and are just about keeping them under 85 celcius. What if it's a hot day and they pop up to 95? Do you panic? Do you call in sick to work? Do you fly back from your holidays?
I am also considering down clocking my GPU's rather than having them maxed out at 830Mhz all the time - a few less hashes isn't going to kill me. With regard to overclocking, I wouldn't consider it. Cards are certainly not designed to be maxed out all the time, never mind overclocking them.
Heat is what matters not load. I overclock my 5970s 37% to 1000 MHz core. You know what temp then run at 24/7? 40 deg C. The fans on the radiator are very low noise too. Most people air cool @ stock and it sounds like a vacuum cleaner. My card runs very cool at 37% higher hashrate and the entire system noise is less than single oscillating fan.
As far as overheating. There is software out there to halt miner, or even shutdown system if temp gets too high.
Now nothing beat the cost of air so if you have a lot of space, don't mind open rigs, and don't care about noise then you should air cool. However if you want high power densities (i.e. 3x 5970 in a quiet closed case) then water cooling can't be beat. Two of my rigs sit in the garage and are air cooled but one of my rigs is also my workstation. It sits indoors in my office. My wife wouldn't be happy to have fans as loud as a a leaf blower running 24/7 (and it still would do a piss poor job of cooling). Even if she didn't mine I couldn't get 3x overclocked 5970s cooled to 40C without water cooling.