Use case: interactive cluster computing from RStudio.
There are many ways to parallelize R, both for interactive and batch-style computation. Different styles include interactive vs. batch, implicit vs. explicit and recasting the problem into a map/reduce framework. The approach shown in this document uses the parallel package and RStudio on Amazon EC2.
For map/reduce style computations, look at Rhipe or Segue. For batch jobs, Starcluster may be a better choice. Brian Holt (Unlicensed) wrote up a document on using R on starcluster called /wiki/spaces/IT/pages/7867417. Other documents give an overview of Distributed Computation Strategy and how to run Distributed Compute Jobs.
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.
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