Index: wp-admin/includes/media.php
===================================================================
--- wp-admin/includes/media.php	(revision 22925)
+++ wp-admin/includes/media.php	(working copy)
@@ -105,6 +105,10 @@
  * @return string the html to insert into editor
  */
 function get_image_send_to_editor($id, $caption, $title, $align, $url='', $rel = false, $size='medium', $alt = '') {
+	if ( ! $alt )
+		$alt = trim( strip_tags( $caption ) );
+	if ( ! $alt )
+		$alt = trim( strip_tags( $title ) );
 
 	$html = get_image_tag($id, $alt, '', $align, $size);