Having working on a Raspberry Pi B+ version, I have found that the real limitation is the memory shortage and CPU performance.
With the release of new Raspberry Pi 2, the little board is equipped with ARM-v7 (ARM Cortex-A7) 900MHz (Quad-core), and 1GB memory (shared with GPU), here is documented specification. By purchasing an MicroSD-HC UHS-I card, we can get a 40MB/s reading speed and around 15-20MB/s writing speed. That is very close to at least a low-end computer.
Therefore, it is possible to create somehow a 3 nodes cluster with low cost (around 200€). The real motivation is to learn how a cluster works and to do some PoC (Proof of Concept) thing.