Index: wp-includes/post.php
===================================================================
--- wp-includes/post.php	(revision 23297)
+++ wp-includes/post.php	(working copy)
@@ -4246,14 +4246,16 @@
  */
 function wp_get_attachment_thumb_file( $post_id = 0 ) {
 	$post_id = (int) $post_id;
-	if ( !$post = get_post( $post_id ) )
+	if ( ! $post = get_post( $post_id ) )
 		return false;
-	if ( !is_array( $imagedata = wp_get_attachment_metadata( $post->ID ) ) )
+	if ( ! is_array( $imagedata = wp_get_attachment_metadata( $post->ID ) ) )
 		return false;
 
 	$file = get_attached_file( $post->ID );
 
-	if ( !empty($imagedata['thumb']) && ($thumbfile = str_replace(basename($file), $imagedata['thumb'], $file)) && file_exists($thumbfile) )
+	if ( ! empty( $imagedata['sizes']['thumbnail']['file'] )
+		&& ( $thumbfile = str_replace( basename( $file ), $imagedata['sizes']['thumbnail']['file'], $file ) )
+		&& file_exists( $thumbfile ) )
 		return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID );
 	return false;
 }