Searching through github ( http://github.com/search?q=puppet) I came across a very similar approach on howto bootstrap EC2 images with capistrano puppet and iclassify. ec2-cap is build on top of the Rightscale Amazon EC2 gem (http://rightaws.rubyforge.org/)

http://github.com/adamhjk/ec2-cap/tree/master

The main idea behind this is to

1. start an EC2 instance (based on a simple declarative description) with capistrano
2. trigger a register process from this instance to the puppet master.
3. sign the certificate on the puppet master
4. pull the default configuration on the newly created host from the puppet master
5. register the new host with the iClassify server
6. tag the new host for the desired purpose (e.g. app for application server)
7. pull the node specific configuration from the puppet master