When I use the tag site_url it should always point to the url I have entered in general config. However Im using a shared SSL, which redirects customers to certain pages like https://secure4.securedomain.com/mydomain_prefix/somepage
In which case, any items using {site_url} are replaced with https://secure4.securedomain.com/
example:
<link href="http://mydomain.com/css/styles.css" rel="stylesheet" type="text/css" />comes out as :
<link href="https://secure4.securedomain.com/css/styles.css" rel="stylesheet" type="text/css" />