Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 15723)
+++ wp-includes/post.php	(working copy)

@@ -3343,10 +3343,13 @@
 	if ( empty($post_date_gmt) )
 		$post_date_gmt = current_time('mysql', 1);
 
-	if ( empty($post_modified) )
-		$post_modified = $post_date;
-	if ( empty($post_modified_gmt) )
+	if ( $update || '0000-00-00 00:00:00' == $post_date ) {
+		$post_modified     = current_time( 'mysql' );
+		$post_modified_gmt = current_time( 'mysql', 1 );
+	} else {
+		$post_modified     = $post_date;
 		$post_modified_gmt = $post_date_gmt;
+	}
 
 	if ( empty($comment_status) ) {
 		if ( $update )