What is the Apple_PubSub_Socket_Render environment variable for?
PubSub is the backend for how Safari and Mail handle RSS feeds.  There are command line utilities too. From the pubsub man page:


  pubsub -- Utility for managing RSS/Atom subscriptions via the PubSub framework


This variable is just part of that process.  

The ability to read RSS feeds in Safari is going away in Mountain Lion, but I don't know if the entire RSS infrastructure is going away too.