Apache mod_rewrite

If you're on an Apache server then you can make a rewrite rule so that when someone tries to visit http://yoursite.com/ Apache would grab the files from http://yoursite.com/forum/

Detailed instructions:
http://httpd.apache.org/docs/current/mod/mod_rewrite.html

This is similar to the rewrite rule you'll need:
http://howto.kryl.info/mod_rewrite/#subdir-as-root

Then you could set up your forum without changing anything.

<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine on
RewriteCond %{REQUEST_URI} !subdir/
RewriteRule (.*) http://example.com/subdir/$1 [L]
</IfModule>