Is there any reason get_query_var isn't a working function in WPMU.
Using the multisite plugin and others, but have a simple page template that - for now - is just trying to test the get query var.
$rss_by_site = get_query_var('this_siteid');
echo '\ '.$rss_by_site;
exit;
And when tested with &this_siteid=3 in the URL, the variable outputs nothing.