Index: wp-includes/post-thumbnail-template.php
===================================================================
--- wp-includes/post-thumbnail-template.php	(revision 17444)
+++ wp-includes/post-thumbnail-template.php	(working copy)
@@ -48,6 +48,37 @@
 }
 
 /**
+ * Update cache for thumbnails in current loop
+ *
+ * @since 3.2
+ */
+function update_thumbnail_cache() {
+	global $wp_query;
+
+	// do this only once per loop
+	if ( $wp_query->_thumbnails_cached )
+		return;
+
+	$thumb_ids = array();
+	foreach ( $wp_query->posts as $post ) {
+		if ( $id = get_post_thumbnail_id( $post->ID ) )
+			$thumb_ids[] = $id;
+	}
+
+	if ( ! empty( $thumb_ids ) ) {
+		get_posts( array(
+			'update_post_term_cache' => false,
+			'include'                => $thumb_ids,
+			'post_type'              => 'attachment',
+			'post_status'            => 'inherit',
+			'nopaging'               => true,
+		) );
+	}
+
+	$wp_query->_thumbnails_cached = true;
+}
+
+/**
  * Retrieve Post Thumbnail.
  *
  * @since 2.9.0
@@ -62,6 +93,10 @@
 	$size = apply_filters( 'post_thumbnail_size', $size );
 	if ( $post_thumbnail_id ) {
 		do_action( 'begin_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size ); // for "Just In Time" filtering of all of wp_get_attachment_image()'s filters
+
+		if ( in_the_loop() )
+			update_thumbnail_cache();
+
 		$html = wp_get_attachment_image( $post_thumbnail_id, $size, false, $attr );
 		do_action( 'end_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size );
 	} else {