OpenID-only login with optional provider restrictions

For sites that want to restrict logins to OpenID-only, it would be practical to have configurable options for hiding the username and password fields, including a "restricted to" field where we could enter domain names for the openID providers we would like to trust.

It is sometimes useful to be able to allow automated account creation and logins based on relationship with only well-known and serious openID providers. In that field one could enter for example yahoo.com, blogspot.com, etc., and this could also be used as the source for a drop-down field in the login/registration form, to simplify things even more for the end users.