James Hamilton highlights the power efficiency of ARM for general purpose web servers. It's almost two years since I gave the first talk on Millicomputing at the HPTS Workshop (which is where I met James for the first time) so its great to see him talking up the principles. He also makes the important point that ARM uses error correcting (ECC) memory, while the Intel Atom doesn't, and thus the Atom is actually less suitable for configuring large numbers in low power enterprise server applications.
The systems at http://www.linux-arm.org/Main/LinuxArmOrg are relatively inefficient blades, they have archaic spinning rust storage attached which must dominate the power consumption. A flash based storage subsystem would make much more sense to me. Web content delivery workloads are very well suited to low cost read-mostly flash storage. They do have a 1.2GHz ARM CPU and 1.5GB of RAM per blade, which is the biggest and fastest ARM configuration I've seen so far.