Index: comment-template.php
===================================================================
--- comment-template.php	(revision 19788)
+++ comment-template.php	(working copy)
@@ -1544,6 +1544,7 @@
 		'title_reply_to'       => __( 'Leave a Reply to %s' ),
 		'cancel_reply_link'    => __( 'Cancel reply' ),
 		'label_submit'         => __( 'Post Comment' ),
+		'tabindex'             => '',
 	);
 
 	$args = wp_parse_args( $args, apply_filters( 'comment_form_defaults', $defaults ) );
@@ -1575,7 +1576,7 @@
 						<?php echo apply_filters( 'comment_form_field_comment', $args['comment_field'] ); ?>
 						<?php echo $args['comment_notes_after']; ?>
 						<p class="form-submit">
-							<input name="submit" type="submit" id="<?php echo esc_attr( $args['id_submit'] ); ?>" value="<?php echo esc_attr( $args['label_submit'] ); ?>" />
+							<input name="submit" type="submit" id="<?php echo esc_attr( $args['id_submit'] ); ?>" value="<?php echo esc_attr( $args['label_submit'] ); ?>" tabindex="<?php echo esc_attr( $args['tabindex'] ); ?>" />
 							<?php comment_id_fields( $post_id ); ?>
 						</p>
 						<?php do_action( 'comment_form', $post_id ); ?>