Using WordPress.com cookies to identify user

Hey,

I was wondering if we could check if a person visiting a WP.ORG blog is in fact a WP.COM user by checking the “wordpress_logged_in” cookie? We could then use an OpenID mechanism to ask the user to log into the WP.ORG blog using the WP.COM identity.

Is this possible? Can the cookie information be used/trusted like this?