Howdy!
We've recently updated our commenting system.
Now, if someone tries to comment with an email address that's attached to a WordPress.com account, they'll need to sign into WordPress.com before they can comment.
If commenters have forgotten their password they can request a reset:
http://wordpress.com/wp-login.php?action=lostpassword
Please don't hesitate to contact our Happiness Engineers if you have any questions. We're glad to help!