Existing Subdomains

How do I implement existing sub domains. So I want forums.domain.com to goto bbpress and not mu. I have this in my .htaccess. With bbpress being a symlink under mu pointing to bbpress

RewriteCond %{HTTP_HOST} forums.domain.com
RewriteCond %{REQUEST_URI} !forums/
RewriteRule ^(.*)$ bbpress/$1 [L]

result is : forums.domain.com/bbpress . I want forums.domain.com .