Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 6178)
+++ wp-includes/pluggable.php	(working copy)
@@ -435,8 +435,9 @@
 	$wpp = parse_url(get_option('home'));
 
 	$allowed_hosts = (array) apply_filters('allowed_redirect_hosts', array($wpp['host']), $lp['host']);
+	$allowed_hosts = array_map('strtolower', $allowed_hosts);
 
-	if ( isset($lp['host']) && !in_array($lp['host'], $allowed_hosts) )
+	if ( isset($lp['host']) && !in_array(strtolower($lp['host']), $allowed_hosts) )
 		$location = get_option('siteurl') . '/wp-admin/';
 
 	wp_redirect($location, $status);