The keyword here is authenticated!
Currently (as far as I know) the de-facto ways to optimize for anonymous access are through static page caches (varnish/squid/boost).
However, as far as I know this doesn't seem to do much good in a scenario where most of your users are authenticated.
Besides the obvious APC, Memcache and throwing money at hardware, what is the best way to performance tune a high-traffic site with mostly authenticated users?
For example: drupal.org, social networks, communities, etc.
I'm more interested in Drupal 6 at the moment so I guess MongoDB doens't apply for now...