We're using Puppet for our deployments, and not just initial deployments, but for code deployments as well. The puppet node classifier talks to a MySQL database

We're using Puppet for our deployments, and not just initial deployments, but for code deployments as well. The puppet node classifier talks to a MySQL database that has the "right" versions for a particular server in it; upgrading consists of changing the version for a server to the desired version, and then running Puppet. I think you could create an EC2 instance that automatically runs Puppet (or Chef) on startup pretty easily. And as someone else mentioned here, you don't have to do a complicated setup with a server to get going, either with Chef or Puppet.