comment_form_title not working with threaded comments

This tag is supposed to display "Reply to (author)," but it's not
working for me and others who have posted on the WordPress forums
without  finding a solution.

Here's the way I coded the tag:
<?php comment_form_title( 'Add <em>your</em> comment', 'Reply to %s' ); ?>

"Add your comment" displays every time, even when the poster is replying
to another comment.

Site link:
http://bit.ly/DBe8x

Background on this issue:

Thread:
http://wordpress.org/support/topic/225225?replies=8#post-1095665

Ticket opened; immediately closed:
http://core.trac.wordpress.org/ticket/8639
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