[Greg]

Interesting… the BuddyPress forums (https://bbpress.org/forums/) have the same problem, but the bbPress.org forums are fine.

I know that the BP forums and our site both use deep integration, so I tried removing that. Result: No more 404.

So it appears that deep integration is part of this problem.

I have bbPress 1.0.2, WPMU 2.8.4a and BuddyPress 1.0.3.