Currently my forum is hosted on forums.sitename.com i want to host it now on sitename.com my forum is getting 5k unique visitors per month from google looking for useful answers. thanks
As long as you set a 304 permanent redirect, then everything should be fine concerning both SEO wise, and visitors.
It all depends, if people are usually directed towards your root and notified they can easily realize the change, also you can put a redirection on the old directory and have them go straight to your new forum area.  <meta HTTP-EQUIV="REFRESH" content="0; url=http://www.yourdomain.com/index.html"> HTML: content="0; = means the number of seconds, so setting to 0 means instantly redirects them.Â
I'm not sure about a 304 redirect. Google recommends a 301 when permanently moving your site. http://www.google.com/support/webmasters/bin/answer.py?answer=83105