diff -r 917dcc6c4ceb -r f088805540ae includes/pageutils.php
--- a/includes/pageutils.php	Sat Nov 17 22:25:37 2007 -0500
+++ b/includes/pageutils.php	Sun Nov 18 20:37:08 2007 -0500
@@ -24,6 +24,7 @@
   function checkusername($name)
   {
     global $db, $session, $paths, $template, $plugins; // Common objects
+    $name = str_replace('_', ' ', $name);
     $q = $db->sql_query('SELECT username FROM ' . table_prefix.'users WHERE username=\'' . $db->escape(rawurldecode($name)) . '\'');
     if ( !$q )
     {