We have been doing this for some time now. Here pictures of our different designs can be found.
This is our first completely fanless design, the previous had one fan in the PSU.
The previous designed use a rather cumbersom Deltatronic PSU, altough fanless it is big and unneccessary powerful for our needs, so in this design a Pico PSU is used instead - leading to a much neater computer.
After successfully building small silent computers, we realized that it would be possible to extend the technology to small, silent personal clusters.
The first one use inexpensive, low power (45 W) dual core athlons on four mainboards. Each of the mainboards are powered by a Pico PSU.
Our first cluster design showed that it is indeed possible to use this cooling technology in small clusters. However, the prototype use desktop components, and we wanted to build something based on "cluster grade" components. So our second design use a Tyan Thunder n3600W (S2935) mainboard with two sockets and Quad-Core AMD Opteron Processor 2376 HE - here AMD Sweden is greatly acknowledged for providing us with the CPUs. This prototype is built for two mainboards, for a total of 16 cores, however, in the tests only one mainboard (8 cores) are used. It is because of this the free side of the cooling channel is isolated - if the temperatures are acceptable in this setup they will also be acceptable with one more mainboard and a "double" channel.
This one uses a slightly different cooling technique. Two large, silent fans are used in order to achieve adequate cooling in a rather small enclosure.
Some other pictures