Index: wp-includes/comment-template.php
===================================================================
--- wp-includes/comment-template.php	(revision 13622)
+++ wp-includes/comment-template.php	(working copy)
@@ -1526,7 +1526,7 @@
 						<?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'] ); ?>" tabindex="<?php echo ( count( $args['fields'] ) + 2 ); ?>" value="<?php echo esc_attr( $args['label_submit'] ); ?>" />
+							<input name="submit" <?php echo esc_attr(apply_filters('comment_form_submit','')); ?> type="submit" id="<?php echo esc_attr( $args['id_submit'] ); ?>" tabindex="<?php echo ( count( $args['fields'] ) + 2 ); ?>" value="<?php echo esc_attr( $args['label_submit'] ); ?>" />
 							<?php comment_id_fields(); ?>
 						</p>
 						<?php do_action( 'comment_form', $post_id ); ?>