[Adam Harley (Kawauso)]

http://phpxref.ftwr.co.uk/wordpress/nav.html?wp-includes/comment-template.php.source.html

In WordPress, comments_popup_link() takes three arguments that tell it how to format the output based on how many comments are present and runs a sprintf() with the appropriate output after retrieving the comment count.

The equivalent to get_comments_number in bbPress would be get_topic_posts, the rest of the function is either WordPress-specific or would need slight tweaking (like what the posts page would actually be).