Well i love watercooling, but as DigiPi said, Dell uses Heatpipes, and as thing progress i would think that most OEM´S will go over to this kind of coolinf solution.
Just look at the lates Zalman Case, totally no fans at all, and everything is cooled by heatpipes and larger heatsinks..
I would think that aircooling will survive the long run and water/phase change etc will still be considered leet
Only diffrence i could see is that aircooling solution are gonna get bigger inn weight and size, so that the surface area is bigger then what todays solutions are.