I didn't find a robots.txt file for bbpress. There are many crawlers looking for this file first so best to have one in your root folder.
The content of my current robots.txt file is now:
User-agent: Mediapartners-Google*
Disallow:
User-agent: *
Disallow: /bb-admin/
Disallow: /bb-cache/
Disallow: /bb-includes/
Disallow: /bb-plugins/
Disallow: /bb-templates/
Disallow: /my-plugins/
Disallow: /my-templates/
Disallow: /plesk-stat/
sitemap: http://forum.go41.de/sitemap.xml
Actually I disallow most of the folders, still would like to disallow 'login'
Sitemap I use is from: http://boakes.org/download/bbpress_sitemap.txt