I've been working on updating a theme to be more 'navigable'. I found the new_topic function could not be used when viewing a topic, so I modified the function to return a proper 'Add New Topic' link when called from a topic view (topic.php)
The code changes can be found in ticket 736:
http://trac.bbpress.org/ticket/736
Here I added a 'New Topic' link in the topic meta section:
http://onthepitch.org/talk/topic/coaching-youth-soccer