bbPress.org

We can see out of the box integration in upcoming releases but that can be done even now although it requires much of manual work.

You can integrate WP + bbPress as per my guide : http://blog.ashfame.com/2009/05/integrate-bbpress-forum-with-your-wordpress-setup/

and using both the registration page wont be a problem but you can redirect it to one if you want. There was a plugin for it, I guess else it can be done manually too.