Session data & sub-domains

[eluser]antiver[/eluser]
I have my domain setup so that any request sent to anything.mydomain.com is handled by a dedicated server, and apache virtual host settings pass all requests to the same CI install.

My goal, is to have addresses like http://antiver.mydomain.com/ be handled by http://mydomain.com/controller/function/antiver

Furthermore, I have my default controller ($route['default_controller']) happily detecting if http://username.mydomain.com/ actually contains a username, and calling the right controller/function to handle the request.

So here's the problem... when you login on http://mydomain.com , and then load http://username.mydomain.com , you're suddenly not logged in anymore. Session data isn't carrying over.

I read this thread-
http://ellislab.com/forums/viewthread/70819/
and tried both:

Code:

$config['cookie_domain'] = "";

(which was the default setting), and:

Code:

$config['cookie_domain'] = ".mydomain.com";

But in either case, the session data is being kept separately. Of course, I need this to work for an unlimited number of sub-domains (for any username.mydomain.com).

Any suggestions appreciated!