Gmail – Is Gmail for Business an openID provider

Facebook uses OAuth, while Google, Yahoo, AOL, and MySpace use OpenID.

For sites that only accept OpenID, you won't be able to use Facebook. Many sites accept both OpenID as well as Facebook, Twitter, LinkedIn, Windows LiveID, etc., that are other non-OpenID solutions.

If you're looking to add support for all these ID systems on your website, you might want to check out Janrain's Engage.

It seems like this was enabled last year. Here is the blog-post:

http://googlecode.blogspot.com/2009/07/google-apps-openid-identity-hub-for.html

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.

The OpenID URL for Google Apps is something like:

https://www.google.com/a/<your-domain>/o8/ud?be=o8