Turns out my problem was rewrite rules from wordpress overwriting bbpress ones. Whoops.
Thanks! I went ahead and put the two as different prefixes. Can you make sure the developers put a note about that in config.php!
I agree with djuggler. I have the same configuration as serimu on my self-hosted server. a .htaccess file at the root (/) for wordpress and a .htaccess file at /forum/ for bbpress. I have AllowOverride All enabled. Yet still no luck. I have had absolutely no problems with my 30+ installs of WordPress. But I’ve had no luck getting permalinks in BBPress working.
This is the only thing holding me back from using BBPress and I’m getting pretty sick of running XDForum.