One thing I have just noticed about phpBB that is loosely tied to mod_rewrite is that when I visit click on the forum index or your logo in the top left it takes me to/index.php, which is identical to forum.modrewrite.com. In some cases a session ID is also attached to this.
Having such duplicate content is not recommended. For example it can split the link popularity that your site may get, with some people linking to the forum subdomain and other linking to the index.php page.
I'm sure you know how to sort this out