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The instances screen will show you that your EC2 instance(s) is (are) now shutting down.
An instance's state will change to terminated once it has been fully shut down and de-provisioned. From this point on, you won't be billed.
You can repeat this entire procedure any time you want to do some Hadoop work. EMR clusters are available on-demand and you can shut them down as soon as you're done using them.
For a more permanent cloud-based cluster, you can provision raw EC2 instances, installing Hadoop on them yourself and federating them into a cluster.
Cloud-based, on-demand Hadoop cluster services comparable to AWS EMR will soon be avaiable from Microsoft, Google and Rackspace, leaving fodder for future galleries here on ZDNet.