Aside from the low-power consumption, another benefit is that the cluster itself is completely silent -- no fans at all or rotating media. However, if you are doing a large cluster backed by an enterprise-grade switch, you may have a fan there, and that can be a bit noisy.
First up are results looking at how well the MPICH2 cluster connected via 10/100 Ethernet is scaling up to 12 ARMv7 cores via the six PandaBoard ES units. Each of the PandaBoard ES had a 16GB SDHC card with a stock install of the OMAP4 version of Ubuntu 12.04 LTS with the stock packages: Linux 3.2, GCC 4.6, and EXT4 file-system as the main components. Again, each PandaBoard ES has an OMAP4460 dual-core Cortex-A9 1.2GHz processor and 1GB of system memory.