Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 21824)
+++ wp-includes/query.php	(working copy)
@@ -1407,7 +1407,7 @@
 		}
 
 		$array_keys = array('category__in', 'category__not_in', 'category__and', 'post__in', 'post__not_in',
-			'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and');
+			'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and', 'post_parent__in', 'post_parent__not_in',);
 
 		foreach ( $array_keys as $key ) {
 			if ( !isset($array[$key]) )
@@ -2168,9 +2168,16 @@
 			$where .= " AND {$wpdb->posts}.ID NOT IN ($post__not_in)";
 		}
 
-		if ( is_numeric($q['post_parent']) )
+		if ( is_numeric($q['post_parent']) ) {
 			$where .= $wpdb->prepare( " AND $wpdb->posts.post_parent = %d ", $q['post_parent'] );
-
+		} elseif ( $q['post_parent__in'] ) {
+			$post_parent__in = implode(',', array_map( 'absint', $q['post_parent__in'] ));
+			$where .= " AND {$wpdb->posts}.post_parent IN ($post_parent__in)";
+		} elseif ( $q['post_parent__not_in'] ) {
+			$post_parent__not_in = implode(',',  array_map( 'absint', $q['post_parent__not_in'] ));
+			$where .= " AND {$wpdb->posts}.post_parent NOT IN ($post_parent__not_in)";
+		}
+		
 		if ( $q['page_id'] ) {
 			if  ( ('page' != get_option('show_on_front') ) || ( $q['page_id'] != get_option('page_for_posts') ) ) {
 				$q['p'] = $q['page_id'];
@@ -2333,6 +2340,8 @@
 			$orderby = '';
 		} elseif ( $q['orderby'] == 'post__in' && ! empty( $post__in ) ) {
 			$orderby = "FIELD( {$wpdb->posts}.ID, $post__in )";
+		} elseif ( $q['orderby'] == 'post_parent__in' && ! empty( $post_parent__in ) ) {
+			$orderby = "FIELD( {$wpdb->posts}.post_parent, $post_parent__in )";
 		} else {
 			// Used to filter values
 			$allowed_keys = array('name', 'author', 'date', 'title', 'modified', 'menu_order', 'parent', 'ID', 'rand', 'comment_count');
Index: wp-includes/class-wp.php
===================================================================
--- wp-includes/class-wp.php	(revision 21824)
+++ wp-includes/class-wp.php	(working copy)
@@ -25,7 +25,7 @@
 	 * @since 2.0.0
 	 * @var array
 	 */
-	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and', 'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and', 'tag_id', 'post_mime_type', 'perm', 'comments_per_page', 'post__in', 'post__not_in');
+	var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and', 'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and', 'tag_id', 'post_mime_type', 'perm', 'comments_per_page', 'post__in', 'post__not_in', 'post_parent__in', 'post_parent__not_in');
 
 	/**
 	 * Extra query variables set by the user.