Passing along a link to this piece in the New York Times regarding liquid immersion cooling, which to date is in use in high performance cooling applications but may eventually move down into the utility-scale data center market.
Though immersion cooling causes shills down the spine of IT managers who didn't have the experience of running water-cooled mainframes from years ago (and there are plenty still out there, the mainframes, I mean!), serious gamers have been dunking their PCs in mineral oil baths for many years.
Although the technology offers clear efficiency advantages (look ma, no fans, anywhere!), I expect it will be used mostly in the super computing arena.
The utility-scale compute environments that I've seen have pretty high energy densities compared to enterprise environments, but they're not pushing 20 kW per rack, which leads you down the path of close-coupled cooling solutions. I'm guessing the energy savings from the 100% outside air cooling schemes used by the big firms will lead them to keep densities low enough to be supported by air-based cooling schemes.