Session problems / where to register

I had a similar question about session scope before, and I learned that if I want a session variable to be available across my subdomains (www.domain.com, my.domain.com, mail.domain.com), I need to register it at domain.com. So I understand that.. The problem I have now is this: I have users who are going to enter the site at my.domain.com. When they hit the index page there, a check is performed against the cookie, and if they are authenticated, it loads their personal index page. But if I want to register session variables here, they are only going to be available to my.domain.com pages, not www.domain.com or mail.domain.com. Short of using a Header call to redirect them to a page on domain.com which just registers the session variables, does anyone have any tricks or advice in tackling this problem? The redirection to set session variables, then redirecting back, is kind of ugly. Thanks, Chad