Massively parallel tasks suffer from the high latency of ethernet networking, which is exacerbated on the Rasperry Pi due to the USB->Ethernet controller. While this is cool in concept, with my own RPis drawing about 2W under load, this entire setup will draw somewhere around 128W, excluding the network fabric and power supply overhead.
At that level of energy consumption, why not just buy a LV or ULV Xeon server and stuff it full of memory? You'll run circles around these Pis, and the cost and energy consumption would probably be comparable.