[laureon]

Are you on v3.5.5 by any chance?

Try this:

<?

require "./auth.php";
require $xcart_dir."/include/categories.php";
$smarty->assign("main", "mainlogin");
$smarty->display("customer/home.tpl");

?>

You will need to have something like this in your 'common_templates' file:

{elseif $main eq "mainlogin"}
{include file="customer/main/mainlogin.tpl"}

hope this helps