Improbulus and I have been doing some Q&A back and forth on just how USM works. From this, I decided to write a couple little scenarios describing exactly what happens when you enable USM on your blog.
Scenarios
When you left-click on a USM capable chicklet, then the Web browser will download the RSS file and try to launch the application associated with the file. Now let's break this into two scenarios where the user has a USM capable client and not.
Scenario #1
If the user has a USM capable client, then it need not be already open. The browser will automagically launch the USM capable client, which may be an RSS reader, but might also just be a stub for transferring the click to My Yahoo or Bloglines. In the end, as long as the USM capable client is installed, does not need to be open, then the user moves toward subscribing to the feed. That's the dream behavior that USM provides.
Scenario #2
If the user does not have a USM capable client, then clicking on the chicklet will prompt the user to save the RSS file locally. About 2 million blogs already have this behavior. This is the same behavior that would happen if you clicked on a Word document and didn't have Word installed. I encourage user's to put a (What is This!) to the right of their RSS chicklet, to guide the users that don't have USM capable clients.
