Google pulled the plug on Reader, but you still want a way to keep the news-reader apps on your various devices in sync, so they all know what feeds you’re subscribed to and which articles you’ve already read.
Here’s what you do. I’ve built this before, as part of the OS X Syndication and PubSub frameworks. (It wasn’t identical to what I’m describing here, partly because app.net didn’t exist so I had to do some clever things with hidden file storage on mac.com. Aren’t you lucky?)