Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 23791)
+++ wp-includes/comment-template.php	(working copy)
@@ -1641,8 +1641,12 @@
 
 	?>
 		<?php if ( comments_open( $post_id ) ) : ?>
-			<?php do_action( 'comment_form_before' ); ?>
-			<div id="respond">
+			<?php 
+			do_action( 'comment_form_before' );
+			$comment_form_classes = apply_filters( 'comment_form_classes', '' );
+			$comment_form_classes = ! empty( $comment_form_classes ) ? sprintf( ' class="%s"', esc_attr( $comment_form_classes ) ) : '';
+			?>
+			<div id="respond"<?php echo $comment_form_classes; ?>>
 				<h3 id="reply-title"><?php comment_form_title( $args['title_reply'], $args['title_reply_to'] ); ?> <small><?php cancel_comment_reply_link( $args['cancel_reply_link'] ); ?></small></h3>
 				<?php if ( get_option( 'comment_registration' ) && !is_user_logged_in() ) : ?>
 					<?php echo $args['must_log_in']; ?>