Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php (revision 12659)
+++ wp-includes/media.php (working copy)
@@ -561,7 +561,7 @@
 		$default_attr = array(
 			'src'	=> $src,
 			'class'	=> "attachment-$size",
-			'alt'	=> trim(strip_tags( $attachment->post_excerpt )),
+			'alt'	=> trim(strip_tags( get_post_meta($attachment_id, '_wp_attachment_image_alt', true) )),
 			'title'	=> trim(strip_tags( $attachment->post_title )),
 		);
 		$attr = wp_parse_args($attr, $default_attr);