Basically Yoast WordPress SEO: XML Sitemaps automatically adds ALL reply url’s to the existing sitemap. This means google will index them all.
> What I’m looking for are where the single reply URL’s are being used in your theme, making them accessible and allowing them to be crawled.
I doubt they are exposed by default, however a quick fix could be adding a rule in robots txt as well as excluding them from your sitemap.
I think there is a bug in the bbp-twentyten files.
I’m talking about archive-forum.php
It calls bbp_get_template_part( 'bbpress/content', 'archive-forums' );
Notice the ‘s’ from archive-forums. That template part doesn’t exist, thus http://www.example.com/forums/ returns empty. (Also themes that don’t have support for bbpress don’t display anything for /forums/ url – is this default functionality?)