Using WP 2.6.2, WP-OpenID 2.2.2, OpenID logins do not work when define('FORCE_SSL_ADMIN', true); is present in wp-config.php.
The login will bring you to the OpenID provider's page, collect credentials, and then return you to to the WordPress login page.
When FORCE_SSL_ADMIN is commented out, WP-OpenID works as expected.
Feel free to try the site, http://www.0x1.net/~cjp , FORCE_SSL_ADMIN is currently on, register permitted.