So, earlier, I wrote about my experience deploying MediaGoblin. None of this was necessary, I was mostly trying to diagnose some problems I was having and found uWSGI a more comfortable environment than FastCGI.
And what problem was this, you ask? Well, when I first ran through the installation instructions, everything worked swimmingly (the port issues mentioned aside). The server it was running on, however, was running Fedora 17, while Fedora 20 has just reached alpha status recently. So, in the interest of retaining compatibility and security fixes, there was a fun afternoon of double distribution updates.
At first, everything seemed to be working fine, but then MediaGoblin began inexplicably throwing 'permission denied' errors. As usually happens in these cases, the culprit was SELinux, an additional security layer which normally transparently protects your system, until something unexpected shows up.
I don't pretend to understand SELinux, but I do understand the security improvements it brings. Plus I don't want to disable it on a system I don't own.
Long, boring story short (and it was very long, being noticed first, and not fixed until well after I figured out how to get uWSGI running), I got something reasonably close to what I think I'm supposed to do. The key was the command setsebool -P httpd_can_network_connect on, which re-enabled the ability for nginx to talk to programs on a network socket, as in the MediaGoblin documentation.
I initially changed to a file-based Unix socket, but I could not, for the life of me figure out how to enable this simply without changing a large number of SELinux booleans. There may be some simpler way of accomplishing this, but alas, in this case SELinux has once again bested me.