Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 21544)
+++ wp-includes/theme.php	(working copy)
@@ -1241,6 +1241,15 @@
 				$args[0] = array_intersect( $args[0], array_keys( get_post_format_slugs() ) );
 			break;
 
+		case 'post-thumbnails' :
+			if ( is_array( $args[0] ) && isset( $_wp_theme_features['post-thumbnails'] ) ) {
+				if ( is_array( $_wp_theme_features['post-thumbnails'][0] ) )
+					$args[0] = array_merge( $_wp_theme_features['post-thumbnails'][0], $args[0] );
+				elseif ( true === $_wp_theme_features['post-thumbnails'] )
+					$args = true;
+			}
+			break;
+
 		case 'custom-header-uploads' :
 			return add_theme_support( 'custom-header', array( 'uploads' => true ) );
 			break;