Euuuh, if you set up a subdomain, you should change your forum path in you ipb settings.
You will have to make a choice. Cause there will always be some users who need to log in in everytime
Or you set up
http://subdomain.host.com as board url, or u enter
http://www.host.com/forumdir/
If you know all your users use the subdomain url, then set the board path to this url
greets