Hi Jonathan,
Have you tried these insructions:
http://people.mozilla.org/~sguha/ec2-installation/I tried it recently, (Oct 02, 2012) and it worked. Always helpful if another pair of eyes tried it out too.
If you have problems following the instructions, post to the group.
Also, though in the example above you have to SSH into the node to run things, it is not required.
If you install RHIPE on your local computer and point HADOOP_HOME, HADOOP_CONF to the whirr location,
you can run RHIPE front end on your local computer and the jobs will run on the nodes.
Cheers
Saptarshi