Re: Integrating WPMU trunk and bbPress RC2
This report sounds similar, although it sounds like they may have been using an earlier RC:
https://bbpress.org/forums/topic/wordpress-mu-271-and-bbpress-10-rc-integration-problem
There was a solution linked from there … hope that points you in the right direction! There was also an integration guide someone created here:
http://umwblogs.org/wiki/index.php/Integrating_WPMu%2C_BuddyPress%2C_and_bbPress
Hope one of those helps!