Four Kitchens maintains and promotes a performance-oriented drupal distro called pressflow. Recently, I noticed they have a branch/fork of pressflow called pressflow-6-evented, and I was all like
"wtf is this? does this bypass the standard drupal bootstrap and path routing shenanigans n shtuff??"
...and ya, it apparently leverages this library (formerly?) known as kargo-event: http://github.com/fhoenig/Kellner#readme
Here are some additional docs from the Four Kitchens wiki on setting up an ubuntu 10.04 system with libevent and the pre-requesites for pressflow-6-evented.
Performance is perpetually the next new frontier!