r-a-y, you’re a lifesaver. I was having the same problem (and pounding my head against the wall) until I found this post. Thanks.
I’m very glad to have found this discussion – after upgrading from WPMU 2.7.something to 2.8.4, the cookie swap stopped working, and I couldn’t find a good explanation anywhere until I found this thread. Upgrading from bbPress 1.0a6 to 1.0.2 fixed everything.
Thanks very much to the developers for playing catchup to WP’s cookie shenanigans.