Re: bbpress and memcache?
Memcache is only useful for multi-server installations otherwise it has too much overhead.
Eaccelerator, APC and Xcache have shared memory solutions that are significantly faster than memcache on a single server setup. But on a single server, it’s also been demonstrated a proper mysql cache can be faster than an object cache in some cases.
Eaccelerator, APC and Xcache can already store server session data in shared memory, you just have to compile with the option and turn it on in PHP.
There are plugins for wordpress that could be ported to bbPress 1.0 for the object cache but IMHO it’s a waste of time unless you have a very slow or overwhelmed mysql server.