Ok, I've just about pulled my last hair over this business. Integration between WPMU & BBPress seems to be working. The problem I can't resolve however is it seems bbpress requires that a new user be a subscriber to the main blog (blogid=1). If that's true, then user is made a "member" in bbpress and they can post just fine.
However, everytime I add a new user/new blog in WPMU the role for that user in the main blog is "No role for this blog". Subsequently, that user can't post in the bbpress forum because they are marked "inactive".
Incidentally, this seems to be exactly what is happen on http://edublogs.org/forums/ because I can't reply to any post or start new ones there.
It seems to work fine here a premium (obviously).
Any ideas what I'm missing?