Re: bbPress creates a back door
Don’t fear. Just change the register.php form in your bbPress template to not have the registration form in it and replace it with a link to your register form on your WPMU install. There are quite a few threads on the bbPress forums about this. If you use the default theme, copy folder to /my-plugins/ and then adjust the theme. That way it won’t break when you upgrade.
Trent