Thanks to a very generous, partial donation by Sony, we have a sixteen PS3 cluster in our department, which we call PS3 Gravity Grid.
We are using "stock" PS3s for this cluster, with no hardware modifications. They are networked together using an inexpensive netgear gigabit switch. For Linux installation, there are several guides available on the internet. For YDL Linux, consider using the guide by Terrasoft Solutions. For Fedora Core 5/6, I found this guide particularly useful. For deploying a parallel job on this cluster, we use a code that implements a standard domain decomposition approach, based on message-passing (MPI). There are more details available on our code below. For compiling, we use GCC and also IBM's XL compilers for the Cell, that are available as part of IBM's Cell SDK. These are available from IBM's alphaworks site. The MPI distribution that we are using is the recently released, OpenMPI distribution for PowerPC Linux.