I put together some code to implement a real bread crumb trail for BBPress that should support any level of subforums. I'll open a ticket for the change too, but until it's in the trunk (hopefully), you can use it by doing the following.
In bb-includes/template-functions.php, just above the // Forum Loop // line, add the following:
(UGH - Why if I put code inside code tags is bbpress insisting on parsing it and tossing in backticks everywhere! To see the code, check this file: http://onthepitch.org/talk/bb-includes/template-functions.txt)
You'll have to update your template, mainly forum.php and topic.php. Look for the line like this"
<h3 class="bbcrumb"><a>"><?php bb_option('name'); ?></a> » <?php forum_name(); ?></h3>
and replace it with something like this:
<h3 class="bbcrumb"><?php forum_bread_crumb(); ?></h3>
And it should work for you.