EDIT: Wow! Double submission! That lack of patience of mine is really working today! Pick the answer that reads best :)
NUTS. bbPress gobbled my reply.
Here goes again:
The <div class="reply"> is getting a border even when there is no text. That's normal since that's what the CSS says. To fix is, wrap the Reply text in a span and style that span.
Backup your theme's files and change the comments.php wp_list_comments() like so:
<?php wp_list_comments(array('type=comment','reply_text'=>'<span class="reply-box">Reply</span>')); ?>
Now modify your CSS to put that styling in the span:
ol.commentlist li div.reply span.reply-box { background:#999; border:1px solid #666; border-radius:2px; -moz-border-radius:2px; -webkit-border-radius:2px; color:#fff; font:bold 9px/1 helvetica,arial,sans-serif; padding:5px 10px; text-align:center; width:36px; }
ol.commentlist li div.reply span.reply-box:hover { background:#2E8FC6; border:1px solid #0F3461; }
ol.commentlist li div.reply span.reply-box a { color:#fff; text-decoration:none; text-transform:uppercase; }
Double check the CSS, I'm shooting from the hip there.
Now when the Reply button is needed, it will wrap in in the span class "reply-box" and style it.
When the button is not needed, you'll be left with an empty
<div class="reply">
</div>
And that's fine. The border is around the span, not the reply class.