I installed bbpress and integrated it with my WPMU setup a few days ago, and I’ve had some users say that they were seeing login weirdness — WPMU requires login to upload an image file (it doesn’t accept), users not able to logout, etc.
It took me a while to get cookies sorted out, but it has been working fine for me in all tests for the past 3 days. In fact, I can’t duplicate anything that is being reported, but I also deleted older cookies a lot.