Done that, been there. Works brilliantly.
1. Install WPMU.
2. Install Buddypress.
3. Change your keys in wp-config.php.
4. Install bbPress with intergrated settings (make sure you use the new keys you made). Make sure you install it in /forums and not in /forum because buddypress will nicely redirect his own /forums to your newly made bbPress install.
5. When everything works good, use this plugin.
Edit: Btw, if you wanna use buddypress login/profiles you can just use redirects.
Worked a charm for me.