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The BioConductor group has put together a Cloud Formation stack for doing interactive parallel computing in R. Follow those instructions, selecting the number of workers and size of the EC2 instances. Once the stack comes up, which took about 10 minutes for me, you log into RStudio on the head node. You'll start R processes on the worker nodes and send commands to the workers.
Note that the parallel package is perfectly happy starting up several copies of R on a single machine, which can be helpful for testing.
Starting a cluster
The IP addresses of the workers (and the head node) get stored on the head node in a file. We'll read that file and create an R process for each core on each worker.
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