Problem:
User can not register when “deep” integration .
bbPress will not have access to WordPress’ functions unless you manually tell bbPress to load WordPress first. This integration step is also known as “deep” integration, it is a totally optional part of integration and is not required to get shared cookies and logins working. In order to do it, you need to put require_once(dirname(__FILE__) . '/path/to/wp-load.php'); in bbPress’ bb-config.php (wp-load.php is in the same directory as WordPress’ wp-config.php file, so if your bbPress folder is directly inside your WordPress folder, the command becomes require_once(dirname(__FILE__) . '/../wp-load.php');).
When i do this,came across a registration problem in bbpress.
Solution:
http://bbpress.org/forums/topic/wp-30-beta-2-integrated-user-registration
The problem is that WordPress declare a global variable called $user_email and in bbPress core file register.php they use the double-dollar notation (variable variables) that are evil. EVIL. So a $$key become a $user_email that is already defined and than not assigned and blah blah blah.