Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 18567)
+++ wp-includes/theme.php	(working copy)
@@ -1935,11 +1935,18 @@
 			// any type is accepted
 			if ( true === $_wp_theme_features[$feature] )  // Registered for all types
 				return true;
-			$content_type = $args[0];
-			if ( in_array($content_type, $_wp_theme_features[$feature][0]) )
+			$content_type  = $args[0];
+			// accept array( 'types' => array( 'post', 'page' ) ), array( 'types' => 'post' ), and
+			// array( 'types' => true ), with a fallback to original array( 'post', 'page' ).
+			if ( isset( $_wp_theme_features[ $feature ][0]['types'] ) )
+				$content_types = $_wp_theme_features[ $feature ][0]['types'];
+			elseif ( isset( $_wp_theme_features[ $feature ][0][0] ) )
+				$content_types = $_wp_theme_features[ $feature ][0];
+			else
 				return true;
-			else
-				return false;
+			if ( true === $content_types )
+				return true;
+			return in_array( $content_type, (array) $content_types );
 			break;
 	}
 
Index: wp-content/themes/twentyeleven/functions.php
===================================================================
--- wp-content/themes/twentyeleven/functions.php	(revision 18567)
+++ wp-content/themes/twentyeleven/functions.php	(working copy)
@@ -107,7 +107,7 @@
 	add_custom_background();
 
 	// This theme uses Featured Images (also known as post thumbnails) for per-post/per-page Custom Header images
-	add_theme_support( 'post-thumbnails' );
+	add_theme_support( 'post-thumbnails', array( 'usage' => __( 'Usage for Twenty Eleven goes here.', 'twentyeleven' ) ) );
 
 	// The next four constants set how Twenty Eleven supports custom headers.
 
Index: wp-admin/includes/meta-boxes.php
===================================================================
--- wp-admin/includes/meta-boxes.php	(revision 18567)
+++ wp-admin/includes/meta-boxes.php	(working copy)
@@ -913,6 +913,9 @@
  */
 function post_thumbnail_meta_box() {
 	global $post;
+	$theme_support = (array) get_theme_support( 'post-thumbnails' );
+	if ( isset( $theme_support[0]['usage'] ) )
+		echo '<p>' . $theme_support[0]['usage'] . '</p>';
 	$thumbnail_id = get_post_meta( $post->ID, '_thumbnail_id', true );
 	echo _wp_post_thumbnail_html( $thumbnail_id );
 }