Google supports OpenID authentication or behaves as openid identity provider, using Google Apps accounts. Basically it means, user can use GA credentials to sign in into different services. This is especially useful for companies to unite other internal services with Google Apps single sign-in point. This is related to Standard edition as well.
I have got it working for PHP. Here is the story.Having spent about 3 hours to get it work, I do not consider OpenID simple protocol that will conquer the Net soon. Why it shouldn't be as simple as adding Paypal payment to a site ?
It does not need any installation, just make "tmp" directory writable. Do not test it using local apache, upload somewhere to the web.
Note: The Federated Login Service is disabled by default for Google Apps Premier and Education Editions. The domain admin can enable it from the Control Panel at http://www.google.com/a/cpanel/<your-domain>/SetupIdp.Test here
UPDATE!! Please look a this post for an updated libraries and example code