Index: wp-includes/canonical.php
===================================================================
--- wp-includes/canonical.php	(revision 15436)
+++ wp-includes/canonical.php	(working copy)
@@ -344,6 +344,14 @@
 	if ( !$redirect_url || $redirect_url == $requested_url )
 		return false;
 
+	// Hex encoded octets are case-insensitive.
+	if ( false !== strpos($requested_url, '%') ) {
+		function lowercase_octets($matches) {
+			return strtolower($matches[0]);
+		}
+		$requested_url = preg_replace_callback('|%[a-fA-F0-9][a-fA-F0-9]|', 'lowercase_octets', $requested_url);
+	}
+
 	// Note that you can use the "redirect_canonical" filter to cancel a canonical redirect for whatever reason by returning FALSE
 	$redirect_url = apply_filters('redirect_canonical', $redirect_url, $requested_url);