Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 4879)
+++ wp-includes/pluggable.php	(working copy)
@@ -161,6 +161,11 @@
 			"From: wordpress@" . preg_replace('#^www\.#', '', strtolower($_SERVER['SERVER_NAME'])) . "\n" . 
 			"Content-Type: text/plain; charset=\"" . get_option('blog_charset') . "\"\n";
 	}
+	
+	// Escape the subject according to the blog's set charset
+	$subject = preg_replace('/([^a-z ])/ie', 'sprintf(\'=%02x\', ord(stripslashes(\'\1\')))', $subject);
+	$subject = str_replace(' ', '_', $subject);
+	$subject = "=?$charset?Q?$subject?=";
 
 	return @mail($to, $subject, $message, $headers);
 }