More writing today, interspersed with bits of procrastination. I started roughing out a standalone OPML Community Server in PHP this afternoon. Who knows how far I'll get, but it has initial bare-bones support for registration, login, and upstreaming. And when I say "support", I don't mean anything grandiose - it's really just a bunch of XML-RPC methods at this point, but it could turn into something interesting.
There is one thing that kind of threw me for a loop in the OPML Editor user interface, though: The "Change Community..." menu item assumes a port of 5337 for any XML-RPC server that's not support.opml.org. That made me have to manually tweak my dotOpmlData.prefs to talk to my own server on port 80.