[dookie]

Change the content of main_chat.php to:

<?php
require("global.php"); 

if( $bbuserid ) { 

$user = $DB_site->query_first( "SELECT username FROM user WHERE userid = $bbuserid" ); 
$bbusername = $user[username]; 
}

eval("dooutput(\"".gettemplate('main_chat')."\");"); 

?>

Now the nickname-field is blank if a user is not registered or nor logged in.
Guests now can chat too.
If you are logged in your nick is set automatically.