https://www.berrange.com/posts/2011/12/18/multi-factor-ssh-authentication-using-yubikey-and-ssh-public-keys-together/
Article on how to use a YubiKey alongside SSH RSA key based login. Bit of a hack, but it appears to work. Nice.
http://www.pyrosoft.co.uk/blog/2010/10/27/import-your-own-rsa-ssh-key-into-amazon-ec2/
How to get custom RSA SSH keypairs into EC2. There is one region missing from this post.