I see it as a good idea, in principle...
So one could still cool the notebook while closed (when working with a cinema display for instance) without limiting the airflow of the machine. The second thing is that the computer could get really quiet (more than today, even if I am already happy with it).
The problem is that any liquid flowing from the bottom casing to the rear screen must have a bendable pipe somewhere below the screen.. (ben once, twice, three times.... it is prone to breaking!).
The second problem I see is the liquid's "inertia" (it must flow in order to dissipate heat). I am no engineer, but if I shut down the computer after intensive use I might get some really hot liquid close to the CPU. This heat will not be dissipated unless the liquid flows.
I think the idea is, in itself, really interesting and might open lots of new possibilities, but on the other hand poses a great amount of challenges.
Having a slightly thicker screen doesn't bother me that much, as its thickness is not percieved when open and, if the weight is well balanced, doesn't create discomfort in use.