I have created an experimental project: running a node.js server + simple MVC CMS on the top of the Raspberry Pi board. Turns out that a $25 computer can actually pull off a decent performance. In fact I have not found a load level where the server would start slowing down, did a 50 concurrent user test on it.
There has been a discussion recently if the ARM based servers could replace conventional Intel based CPUs in the future. As ridiculous as it sounds to build servers off phone chips, I imagine that it is the same like if somebody in the 80's said that we would be running our server farms of PCs.
Here is another thing: joyent.com which is the main node.js hosting provider actually runs their servers on a minimal 250k memory VM instances and just scales by turning on more of these as needed. Why could not we do the same thing on per-board basis, and this way eliminate hardware failure as well as power consumption.