I visited a user today who was having issues.

He's using IE.

What I found was, if he's logged onto the board at wwww.mydomain.com/forums/ubbthreads, and then visits a page at mydomain.com/forums/ubbthreads (without the "www") he shows as not logged in. He has to log-in again while on the non-www url.

In my domains for HTTP Referer Check, I have all of these set:
http://www.mydomain.com
http://mydomain.com
www.mydomain.com
mydomain.com

This wouldn't be such a big deal, but I have parts of my site that users have to be logged in to visit. For whatever reason, sometimes people will email links without the "www"

Is there something I can adjust on my end so that when someone is logged in (no matter what URL they started at) they can jump around to both versions of the URL and still show as logged in?