The instructions for adding a public key to the authorized_keys file are a bit reckless, as they will wipe out any others that exist. Although not universal (not included with Mac OS X, for example) the ssh-copy-id command is much preferable. At its most basic:

  ssh-copy-id example.com

Or, to specify a specific key & remote user:

  ssh-copy-id -i ~/.ssh/id_rsa.pub bob@example.com

This will properly append the key, with the bonus of securing the permissions of the remote user's home, ~/.ssh, and ~/.ssh/authorized_keys.