As I think a sitemap.xml is important for any website I got the latest sitemap plugin for bbpress here:
http://plugins-dev.bbpress.org/browser/bbpress-sitemap-generator/trunk/bbpress_sitemap.php
I edited this bbpress_sitemap.php on line 16 to look like this:
$sitemap_file = $_SERVER['DOCUMENT_ROOT']."/sitemap.xml";
as my bbpress is installed in the root folder of a sub-domain, the sitemap.xml should also be in the root.
After changing the reference to the location of this file (sitemap.xml) I uploaded bbpress_sitemap.php to the new created my-plugins folder.
Before enabling it I created an empty file sitemap.xml in the root of the forum and set the rights to 777.
I enabled the plugin and to trigger the sitemap creation just opened one post and saved it. With FileZilla ftp client I could see that the previously empty file now increased in size - it was successfully created
Still get problems with empty links to tag pages however...