Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 6640)
+++ wp-includes/post.php	(working copy)
@@ -1062,7 +1062,7 @@
 			$post_date_gmt = '0000-00-00 00:00:00';
 	}
 
-	if ( $update ) {
+	if ( $update || '0000-00-00 00:00:00' == $post_date ) {
 		$post_modified     = current_time( 'mysql' );
 		$post_modified_gmt = current_time( 'mysql', 1 );
 	} else {