It's nice that the support is there, but are there any sites actually using it? My concern is that default matter - ie, if you make it easier to just support a few fixed servers, that's what will generally happen.
When you login to google, it prompts you for an email address first. If your email is associated with an organization that has configured Google Apps to use their OP for authentication, Google will redirect the user to their home domain based on the email.