How's your German? ;-)
There's a thread over at geeklog.info explaining how to do that.
The solution is pretty simple, actually, and relies on the fact that you can use PHP in header.thtml:
Text Formatted Code
<?php
if ($_USER['uid'] > 1) {
print '<a href="' . $_CONF['site_url'] . '/users.php?mode=logout">Logout</a>';
} else {
print '<a href="' . $_CONF['site_url'] . '/users.php?mode=login">Login</a>';
}
?>
bye, Dirk