Possibly domain name is a factor in how highly you rank (or perhaps even subdomain - is <a href="http://forum.yoursite.com">http://forum.yoursite.com</a> going to rate higher or lower than <a href="http://www.yoursite.com/forum">http://www.yoursite.com/forum</a> on a search for your keyword forum assuming all other things are equal?) but I feel it is an almost non-existent factor.