Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 21078)
+++ wp-includes/media.php	(working copy)
@@ -649,7 +649,7 @@
 function wp_get_attachment_image($attachment_id, $size = 'thumbnail', $icon = false, $attr = '') {
 
 	$html = '';
-	$image = wp_get_attachment_image_src($attachment_id, $size, $icon);
+	$image = apply_filters( 'wp_get_attachment_image_src', wp_get_attachment_image_src($attachment_id, $size, $icon), $attachment_id, $size, $icon );
 	if ( $image ) {
 		list($src, $width, $height) = $image;
 		$hwstring = image_hwstring($width, $height);