bbPress.org

Where have you installed WordPress and bbPress relative to each other, and what actually are your .htaccess settings for WordPress?

Mine is just:

# BEGIN WordPress

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule . /index.php [L]

# END WordPress

which seems to hand over nicely to the forums that are under /forums/ with their own .htaccess file