Well, it's been a wild webhosting ride recently. I've been automating dev/staging/live production procedures with my Drupal and Magento clients. In a few days I'll publish a comprehensive blogpost about this whole project, replete with nginx, php, php-fpm, etc configuration goodies.
For now, here's one tidbit that I just can't hold back:
I ran into an interesting problem with running multiple Magento sites on the same server: the new Magento devsite I set up would unfailingly read the old site's db configuration information (from app/etc/local.xml) 9/10 times after restarting nginx and php5-fpm. And to make things even more confusing, on some restarts, it'd read the devsite db configuration and so would the live site!
After trying every single possible iteration of php5-fpm and nginx options that I thought might be relevant, I remembered that I had set up Magento to use APC for caching, and changed the prefix:
apc alphanumeric (i.e. change this to something else)
It took a LONG time to track down this issue. I had forgotten that I had apc cache running on this site, and I never would've guessed that it would mix up different site's caches. The search was even more complicated because I am not as confident with my nginx/php-fpm skills as i am with apache/mod_php. Serves me right for blindly implementing the magento apc cache without understanding what the various xml config options mean.
After finding the problem on my own, I searched to see if anyone else has experienced this and found a post on the magento board. Wish I'd seen that 6 hours ago.
I'm going to add an APC-prefix-changing sed operation to my magento dev/staging/production scripts now.