I noticed that you were interested in adding memcache support. There's a very very easy way to do this in Ubuntu and maybe Debian (not sure about other distros):
Just put your page cache somewhere inside /dev/shm. Read more about it here: http://www.cyberciti.biz/tips/what-is-devshm-and-its-practical-usage.html
Very easy to understand and implement!
Yes you can do it with Nginx based file cache: mount the cache dir on /dev/shm using tmpfs. But by memcache I mean the backend that Nginx provides. It can work as a first or a second layer after the Nginx cache. Also it automatically fixes the issues with expirations. What is missing is some time to core dump the memcache and see how the keys are built by the Drupal memcache backend and do it on the config.
I'm leaving this open as reminder.
Why would you want to have multiple layers of caching in this case? Having a microcache in /dev/shm should be plenty fast enough for just about any site. That's one of the things that we did on Gamespot.com, IIRC.
Yes. I've done it also before. But if you have multiple layers you can have staggered TTLs that puts you closer to the goal of almost zero load on PHP. I've used it before where there's two layer microcaching: