Re: bbpress and memcache?
We don’t use memcache to store user sessions, user sessions are stored in the browser cookie.
There is a memcache capable object cache though which is built in to bbPress via BackPress. To enable it (with default memcache settings) all you need to do is add this to your bb-config.php:
define( 'BB_OBJECT_CACHE_TYPE', 'memcached' );
Note that this will not work if you are using the memcache object cache in WordPress, and you have WordPress deep integrated into bbPress.