A customer asked me to help set up a Rails app on EC2 today and I must say I am impressed how well Amazon supports developers using their web services. Setting up a new Amazon account, generating the required keys, starting with an operating system image that roughly matches what you need to work on (e.g., Java, Ruby, Perl, etc.) and getting up and running is basically a one hour job.
Amazon's developer services are awesome, especially because of the quality of documentation and tools. They did a very good job!
I am working on a Ruby Web 3.0 developer's book and a small part of it deals with deployment strategies. I am definitely going to add a section on EC2.