Re: Integration problems
The plugins, especially bbpress-integration (goes in WP plugins and has option page once installed to choose bb_ as database prefix), tells bbPress to use the WordPress users. Therefore, if you login to each program (either/or), it uses the WP users. All registrations in bbPress make users in WP users. It is simple.
As for the login working in both, the settings in config.php in bbPress tells bbPress to use the WP cookies so they share the same one. Therefore, both programs know to use the same user without having to login again.