Q:

How do I build a cluster?

There have been a few articles floating around online over the last few years about building a cluster of computers at home - here for example.
The Pi strikes me as a good platform for building / testing this kind of thing due to their low cost; should "generic" guides like this transfer over to the Pi pretty easily, or is there anything specifically I should be aware of when attempting such a project?

A:

I suggest looking at Dispy - the distributed computation python module. 
To run a program on a number of Raspberry Pi's (nodes) from a PC (server - assume IP is 192.168.0.100):

Install an operating system on each RasPi
Attach each RasPi to your network. Find the IP (if dynamic), or set up static IPs.
(Let's assume that you have three nodes, and their IPs are 192.168.0.50-52)
Set up Python (if not already), install dispy, then run dispynode.py -i 192.168.0.100 on each RasPi. This will tell dispynode to receive job information from the server.
On the PC (the server), install dispy, then run the following python code:  

#!/usr/bin/env python
import dispy
cluster = dispy.JobCluster('/some/program', nodes=['192.168.0.50', '192.168.0.51', '192.168.0.52'])

You can also replace /some/program with a python function - e.g. compute.
You can also include dependencies such as python objects, modules and files (which dispy will transfer to each node) by adding depends=[ClassA, moduleB, 'file1']

A:

You should be aware of the work that has already been done - there's even a name for a cluster of RasPi boxen. The Embedded Linux Wiki says a Bramble is defined as "a Beowulf cluster of Raspberry Pi devices". Raspberry Pi Homebrew has a number of posts about Brambles, and see also the Foundation's own forum.

A:

Some guys at Southampton Uni have put together a cluster and written a detailed overview of their work at http://www.southampton.ac.uk/~sjc/raspberrypi/.

