I understood that the first poster wants to have "
www.sitename.com" running Subdreamer and "forums.sitename.com" running the forum.
This cannot be done in the way you described, because Subdreamer part of the website does not work.
I have done this modification on the website I run. In order to do it, modifications are needed to Subdreamer code.
First, you need to alter the usersystem code to change the "cookie domain" to highest common domain for the site, for example ".sitename.com". This way, the forum and Subdreamer will use the same cookies for session management. This also requires setting the cookie domain in the forum. If cookie domain is not set, sessions across forum and Subdreamer won't work.
Then, you need to change plug-ins that deal with forum functions so that references to "$sdurl . $forumpath" are replaced with the forum domain.
There could be some problems with security implications when the cookie domain is set. I remember reading a specification about how browsers should act when cookie domain is set. The point was, that when the cookie domain is "too broad", like ".com.tw", it means that the browser sends the cookie to all sites that end with .com.tw. This is a security risk, and the document proposed that the browser wouldn't accept this kind of broad cookie domains.
In conclusion, this is possible, but needs changing Subdreamer code. I'm sure this could be added as a feature to Subdreamer. I don't know if developers are willing to implement this though.
On my site this setup works fine, but occasionally the auto-login feature seems to work incorrectly with Opera browser. This could occur because Opera might have more strict policy on cookies.