Drupal as a SSO server?

I've been moving a non-profit to Google apps for email hosting, and after reading this post about drupal as an http auth server, I had a similar thought. Has anyone taken a try at implementing Drupal as a Single Sign On (SSO) server using SAML (Security Authentication Markup Language)?

Google Apps supports external authentication this way, but I haven't found any modules that look like they'd do this. Maybe a good GHOP project?