Thank you very much for your reply. This isn't urgent for me, I have made the PGRMS yum repo work with the Amazon Linux AMI by extracting the .repo file and GPG key, hardcoding the version to RHEL-5, and installing them manually. I've created an Opscode Chef recipe that automatically installs them on my instances (https://github.com/hgavin/chef-pgdg_90_amazon_ami).
There are a couple of Postgres dependencies that aren't available from the Amazon Yum repo or EPEL. When installing postgresql 9.0 from pgrms I first have to install libtermcap and termcap from the CentOS repo. This step is in my Chef recipe for Postgresql (https://github.com/hgavin/chef-postgres_pgdg_90).